diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 318ba96605..3813792418 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3921,9 +3921,11 @@ nvim_open_win({buffer}, {enter}, {config})                   *nvim_open_win()*
                     Default is `"left"`.
                   • vertical: Split vertically |:vertical|.
                   • width: Window width (in character cells). Minimum of 1.
-                  • win: |window-ID| window to split, or relative window when
-                    creating a float (relative="win"). When splitting,
-                    negative value works like |:topleft|, |:botright|.
+                  • win: |window-ID| target window. Can be in a different tab
+                    page. Determines the window to split (negative values act
+                    like |:topleft|, |:botright|), the relative window for a
+                    `relative="win"` float, or just the target tab page
+                    (inferred from the window) for others.
                   • zindex: Stacking order. floats with higher `zindex` go on
                     top on floats with lower indices. Must be larger than
                     zero. The following screen elements have hard-coded
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index e8c52a8ca9..7e5f7565b1 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -167,6 +167,7 @@ API
   they were so specified in `nvim_create_user_command()`.
 • |nvim_open_win()| floating windows can show a 'statusline'. Plugins can use
   `style='minimal'` or `:setlocal statusline=` to hide the statusline.
+• |nvim_win_set_config()| can move windows to other tab pages as floats.
 • Added experimental |nvim__exec_lua_fast()| to allow remote API clients to
   execute code while nvim is blocking for input.
 • |vim.secure.trust()| accepts `path` for the `allow` action.
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index 975db4ac3b..0510e2bef9 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -1848,8 +1848,9 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---     Default is `"left"`.
 --- - vertical: Split vertically `:vertical`.
 --- - width: Window width (in character cells). Minimum of 1.
---- - win: `window-ID` window to split, or relative window when creating a float (relative="win").
----     When splitting, negative value works like `:topleft`, `:botright`.
+--- - win: `window-ID` target window. Can be in a different tab page. Determines the window to
+---     split (negative values act like `:topleft`, `:botright`), the relative window for a
+---     `relative="win"` float, or just the target tab page (inferred from the window) for others.
 --- - zindex: Stacking order. floats with higher `zindex` go on top on
 ---             floats with lower indices. Must be larger than zero. The
 ---             following screen elements have hard-coded z-indices:
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 55e089b133..921a622c6e 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -18,6 +18,8 @@
 #include "nvim/drawscreen.h"
 #include "nvim/errors.h"
 #include "nvim/eval/window.h"
+#include "nvim/ex_cmds_defs.h"
+#include "nvim/ex_docmd.h"
 #include "nvim/globals.h"
 #include "nvim/highlight_group.h"
 #include "nvim/macros_defs.h"
@@ -32,6 +34,7 @@
 #include "nvim/syntax.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
+#include "nvim/ui_compositor.h"
 #include "nvim/ui_defs.h"
 #include "nvim/vim_defs.h"
 #include "nvim/window.h"
@@ -39,6 +42,8 @@
 
 #include "api/win_config.c.generated.h"
 
+#define HAS_KEY_X(d, key) HAS_KEY(d, win_config, key)
+
 /// Opens a new split window, floating window, or external window.
 ///
 /// - Specify `relative` to create a floating window. Floats are drawn over the split layout,
@@ -177,8 +182,9 @@
 ///       Default is `"left"`.
 ///   - vertical: Split vertically |:vertical|.
 ///   - width: Window width (in character cells). Minimum of 1.
-///   - win: |window-ID| window to split, or relative window when creating a float (relative="win").
-///       When splitting, negative value works like |:topleft|, |:botright|.
+///   - win: |window-ID| target window. Can be in a different tab page. Determines the window to
+///       split (negative values act like |:topleft|, |:botright|), the relative window for a
+///       `relative="win"` float, or just the target tab page (inferred from the window) for others.
 ///   - zindex: Stacking order. floats with higher `zindex` go on top on
 ///               floats with lower indices. Must be larger than zero. The
 ///               following screen elements have hard-coded z-indices:
@@ -197,7 +203,6 @@
 Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Error *err)
   FUNC_API_SINCE(6) FUNC_API_TEXTLOCK_ALLOW_CMDWIN
 {
-#define HAS_KEY_X(d, key) HAS_KEY(d, win_config, key)
   buf_T *buf = find_buffer_by_handle(buffer, err);
   if (!buf) {
     return 0;
@@ -348,7 +353,6 @@ cleanup:
     unblock_autocmds();
   }
   return rv;
-#undef HAS_KEY_X
 }
 
 static WinSplit win_split_dir(win_T *win)
@@ -381,10 +385,56 @@ static int win_split_flags(WinSplit split, bool toplevel)
   return flags;
 }
 
-static bool win_config_split(win_T *win, Dict(win_config) *config, WinConfig *fconfig, Error *err)
+/// Checks if window `wp` can be moved to tabpage `tp`.
+static bool win_can_move_tp(win_T *wp, tabpage_T *tp, Error *err)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (one_window(wp, tp == curtab ? NULL : tp)) {
+    api_set_error(err, kErrorTypeException, "Cannot move last non-floating window");
+    return false;
+  }
+  // Like closing, moving windows between tabpages makes win_valid return false. Helpful when e.g:
+  // walking the window list, as w_next/w_prev can unexpectedly refer to windows in another tabpage!
+  // Check related locks, in case they were set to avoid checking win_valid.
+  if (win_locked(wp)) {
+    api_set_error(err, kErrorTypeException, "Cannot move window to another tabpage whilst in use");
+    return false;
+  }
+  if (window_layout_locked_err(CMD_SIZE, err)) {
+    return false;  // error already set
+  }
+  if (textlock || expr_map_locked()) {
+    api_set_error(err, kErrorTypeException, "%s", e_textlock);
+    return false;
+  }
+  if (is_aucmd_win(wp)) {
+    api_set_error(err, kErrorTypeException, "Cannot move autocmd window to another tabpage");
+    return false;
+  }
+  // Can't move the cmdwin or its old curwin to a different tabpage.
+  if (wp == cmdwin_win || wp == cmdwin_old_curwin) {
+    api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+    return false;
+  }
+  return true;
+}
+
+static win_T *win_find_altwin(win_T *win, tabpage_T *tp)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (win->w_floating) {
+    return win_float_find_altwin(win, tp == curtab ? NULL : tp);
+  } else {
+    int dir;
+    return winframe_find_altwin(win, &dir, tp == curtab ? NULL : tp, NULL);
+  }
+}
+
+/// Configures `win` into a split, also moving it to another tabpage if requested.
+static bool win_config_split(win_T *win, const Dict(win_config) *config, WinConfig *fconfig,
+                             Error *err)
   FUNC_ATTR_NONNULL_ALL
 {
-#define HAS_KEY_X(d, key) HAS_KEY(d, win_config, key)
   bool was_split = !win->w_floating;
   bool has_split = HAS_KEY_X(config, split);
   bool has_vertical = HAS_KEY_X(config, vertical);
@@ -423,14 +473,8 @@ static bool win_config_split(win_T *win, Dict(win_config) *config, WinConfig *fc
       api_set_error(err, kErrorTypeException, "Cannot split a floating window");
       return false;
     }
-    if (is_aucmd_win(win) && win_tp != parent_tp) {
-      api_set_error(err, kErrorTypeException, "Cannot move autocmd window to another tabpage");
-      return false;
-    }
-    // Can't move the cmdwin or its old curwin to a different tabpage.
-    if ((win == cmdwin_win || win == cmdwin_old_curwin) && win_tp != parent_tp) {
-      api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
-      return false;
+    if (win_tp != parent_tp && !win_can_move_tp(win, win_tp, err)) {
+      return false;  // error already set
     }
   }
 
@@ -443,24 +487,13 @@ static bool win_config_split(win_T *win, Dict(win_config) *config, WinConfig *fc
   // window list or remove its frame (if non-floating), so it's valid for autocommands.
   const bool curwin_moving_tp = win == curwin && parent && win_tp != parent_tp;
   if (curwin_moving_tp) {
-    if (was_split) {
-      int dir;
-      win_T *altwin = winframe_find_altwin(win, &dir, NULL, NULL);
-      // Autocommands may still make this the last non-float after this check.
-      // That case will be caught later when trying to move the window.
-      if (!altwin) {
-        api_set_error(err, kErrorTypeException, "Cannot move last non-floating window");
-        return false;
-      }
-      win_goto(altwin);
-    } else {
-      win_goto(win_float_find_altwin(win, NULL));
-    }
+    win_T *altwin = win_find_altwin(win, win_tp);
+    assert(altwin);  // win_can_move_tp ensures `win` is not the only window
+    win_goto(altwin);
 
     // Autocommands may have been a real nuisance and messed things up...
     if (curwin == win) {
-      api_set_error(err, kErrorTypeException, "Failed to switch away from window %d",
-                    win->handle);
+      api_set_error(err, kErrorTypeException, "Failed to switch away from window %d", win->handle);
       return false;
     }
     win_tp = win_find_tabpage(win);
@@ -608,7 +641,98 @@ resize:
   }
   merge_win_config(&win->w_config, *fconfig);
   return true;
-#undef HAS_KEY_X
+}
+
+/// Configures `win` into a float, also moving it to another tabpage if requested.
+static bool win_config_float_tp(win_T *win, const Dict(win_config) *config,
+                                const WinConfig *fconfig, Error *err)
+  FUNC_ATTR_NONNULL_ALL
+{
+  tabpage_T *win_tp = win_find_tabpage(win);
+  win_T *parent = win;
+  tabpage_T *parent_tp = win_tp;
+  if (HAS_KEY_X(config, win)) {
+    parent = find_window_by_handle(fconfig->window, err);
+    if (!parent) {
+      return false;  // error already set
+    }
+    parent_tp = win_find_tabpage(parent);
+  }
+
+  bool curwin_moving_tp = false;
+  win_T *altwin = NULL;
+
+  if (win_tp != parent_tp) {
+    if (!win_can_move_tp(win, win_tp, err)) {
+      return false;  // error already set
+    }
+    altwin = win_find_altwin(win, win_tp);
+    assert(altwin);  // win_can_move_tp ensures `win` is not the only window
+
+    // If we are moving curwin to another tabpage, switch windows *before* we remove it from the
+    // window list or remove its frame (if non-floating), so it's valid for autocommands.
+    if (curwin == win) {
+      curwin_moving_tp = true;
+      win_goto(altwin);
+
+      // Autocommands may have been a real nuisance and messed things up...
+      if (curwin == win) {
+        api_set_error(err, kErrorTypeException, "Failed to switch away from window %d",
+                      win->handle);
+        return false;
+      }
+      win_tp = win_find_tabpage(win);
+      parent_tp = win_find_tabpage(parent);
+
+      if (!win_tp || !parent_tp) {
+        api_set_error(err, kErrorTypeException, "Target windows were closed");
+        goto restore_curwin;
+      }
+      if (win_tp != parent_tp && !win_can_move_tp(win, win_tp, err)) {
+        goto restore_curwin;  // error already set
+      }
+      altwin = win_find_altwin(win, win_tp);
+      assert(altwin);  // win_can_move_tp ensures `win` is not the only window
+    }
+  }
+
+  // Convert the window to a float if needed.
+  if (!win->w_floating) {
+    if (!win_new_float(win, false, *fconfig, err)) {
+restore_curwin:
+      // If `win` was the original curwin, and autocommands didn't move it outside of curtab, be a
+      // good citizen and try to return to it.
+      if (curwin_moving_tp && win_valid(win)) {
+        win_goto(win);
+      }
+      return false;
+    }
+    redraw_later(win, UPD_NOT_VALID);
+  }
+
+  if (win_tp != parent_tp) {
+    win_remove(win, win_tp == curtab ? NULL : win_tp);
+    tabpage_T *append_tp = parent_tp == curtab ? NULL : parent_tp;
+    win_append(lastwin_nofloating(append_tp), win, append_tp);
+
+    // If `win` was the curwin of its old tabpage, select a new curwin for it.
+    if (win_tp != curtab && win_tp->tp_curwin == win) {
+      win_tp->tp_curwin = altwin;
+    }
+
+    // Remove grid if present. More reliable than checking curtab, as tabpage_check_windows may not
+    // run when temporarily switching tabpages, meaning grids may be stale from another tabpage!
+    // (e.g: switch_win_noblock with no_display=true)
+    ui_comp_remove_grid(&win->w_grid_alloc);
+
+    // Redraw tabline, update window's hl attribs, etc. Set must_redraw here, as redraw_later might
+    // not if w_redr_type >= UPD_NOT_VALID was set in the old tabpage.
+    redraw_later(win, UPD_NOT_VALID);
+    set_must_redraw(UPD_NOT_VALID);
+  }
+
+  win_config_float(win, *fconfig);
+  return true;
 }
 
 /// Reconfigures the layout and properties of a window.
@@ -631,7 +755,6 @@ resize:
 void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
   FUNC_API_SINCE(6)
 {
-#define HAS_KEY_X(d, key) HAS_KEY(d, win_config, key)
   win_T *win = find_window_by_handle(window, err);
   if (!win) {
     return;
@@ -645,24 +768,21 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
   WinConfig fconfig = win->w_config;
 
   bool to_split = config->relative.size == 0
-                  && !(HAS_KEY_X(config, external) ? config->external : fconfig.external)
+                  && !(HAS_KEY_X(config, external) && config->external)
                   && (has_split || has_vertical || was_split);
 
   if (!parse_win_config(win, config, &fconfig, !was_split || to_split, err)) {
     return;
   }
 
-  if (was_split && !to_split) {
-    if (!win_new_float(win, false, fconfig, err)) {
-      return;
-    }
-    redraw_later(win, UPD_NOT_VALID);
-  } else if (to_split) {
+  if (to_split) {
     if (!win_config_split(win, config, &fconfig, err)) {
       return;
     }
   } else {
-    win_config_float(win, fconfig);
+    if (!win_config_float_tp(win, config, &fconfig, err)) {
+      return;
+    }
   }
 
   if (fconfig.style == kWinStyleMinimal && old_style != fconfig.style) {
@@ -675,7 +795,6 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
   } else if (win == cmdline_win && fconfig._cmdline_offset == INT_MAX) {
     cmdline_win = NULL;
   }
-#undef HAS_KEY_X
 }
 
 #define PUT_KEY_X(d, key, value) PUT_KEY(d, win_config, key, value)
@@ -1144,7 +1263,6 @@ bool parse_winborder(WinConfig *fconfig, char *border_opt, Error *err)
 static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fconfig, bool reconf,
                              Error *err)
 {
-#define HAS_KEY_X(d, key) HAS_KEY(d, win_config, key)
   bool has_relative = false, relative_is_win = false, is_split = false;
   if (config->relative.size > 0) {
     if (!parse_float_relative(config->relative, &fconfig->relative)) {
@@ -1167,6 +1285,7 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
   } else if (!config->external) {
     if (HAS_KEY_X(config, vertical) || HAS_KEY_X(config, split)) {
       is_split = true;
+      fconfig->external = false;
     } else if (wp == NULL) {  // new win
       api_set_error(err, kErrorTypeValidation,
                     "Must specify 'relative' or 'external' when creating a float");
@@ -1258,36 +1377,6 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
     goto fail;
   }
 
-  if (relative_is_win || is_split) {
-    if (reconf && relative_is_win) {
-      win_T *target_win = find_window_by_handle(config->win, err);
-      if (!target_win) {
-        goto fail;
-      }
-
-      if (target_win == wp) {
-        api_set_error(err, kErrorTypeException, "floating window cannot be relative to itself");
-        goto fail;
-      }
-    }
-    fconfig->window = curwin->handle;
-    if (HAS_KEY_X(config, win)) {
-      if (config->win > 0) {
-        fconfig->window = config->win;
-      }
-    }
-  } else if (HAS_KEY_X(config, win)) {
-    if (has_relative) {
-      api_set_error(err, kErrorTypeValidation,
-                    "'win' key is only valid with relative='win' and relative=''");
-      goto fail;
-    } else if (!is_split) {
-      api_set_error(err, kErrorTypeValidation,
-                    "non-float with 'win' requires at least 'split' or 'vertical'");
-      goto fail;
-    }
-  }
-
   if (HAS_KEY_X(config, external)) {
     fconfig->external = config->external;
     if (has_relative && fconfig->external) {
@@ -1301,6 +1390,38 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
     }
   }
 
+  if (HAS_KEY_X(config, win) && fconfig->external) {
+    api_set_error(err, kErrorTypeValidation, "external window cannot have 'win'");
+    goto fail;
+  }
+  if (relative_is_win || (HAS_KEY_X(config, win) && !is_split && wp && wp->w_floating
+                          && fconfig->relative == kFloatRelativeWindow)) {
+    // When relative=win is given, missing win field means win=0.
+    win_T *target_win = find_window_by_handle(config->win, err);
+    if (!target_win) {
+      goto fail;
+    }
+    if (target_win == wp) {
+      api_set_error(err, kErrorTypeException, "floating window cannot be relative to itself");
+      goto fail;
+    }
+    fconfig->window = target_win->handle;
+  } else {
+    // Handle is not validated here, as win_config_split can accept negative values.
+    if (HAS_KEY_X(config, win)) {
+      if (!is_split && !has_relative && (!wp || !wp->w_floating)) {
+        api_set_error(err, kErrorTypeValidation,
+                      "non-float with 'win' requires at least 'split' or 'vertical'");
+        goto fail;
+      }
+      fconfig->window = config->win;
+    }
+    // Resolve, but skip validating. E.g: win_config_split accepts negative "win".
+    if (fconfig->window == 0) {
+      fconfig->window = curwin->handle;
+    }
+  }
+
   if (HAS_KEY_X(config, focusable)) {
     fconfig->focusable = config->focusable;
     fconfig->mouse = config->focusable;
@@ -1422,5 +1543,4 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
 fail:
   merge_win_config(fconfig, wp != NULL ? wp->w_config : WIN_CONFIG_INIT);
   return false;
-#undef HAS_KEY_X
 }
diff --git a/src/nvim/arglist.c b/src/nvim/arglist.c
index 1d5903aefc..7d12c92555 100644
--- a/src/nvim/arglist.c
+++ b/src/nvim/arglist.c
@@ -1116,7 +1116,7 @@ static void do_arg_all(int count, int forceit, int keep_tabs)
   last_curwin = curwin;
   last_curtab = curtab;
   // lastwin may be aucmd_win
-  win_enter(lastwin_nofloating(), false);
+  win_enter(lastwin_nofloating(NULL), false);
 
   // Open up to "count" windows.
   arg_all_open_windows(&aall, count);
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 373cc6263b..94b670b6dc 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -3692,7 +3692,7 @@ void ex_buffer_all(exarg_T *eap)
   // Don't execute Win/Buf Enter/Leave autocommands here.
   autocmd_no_enter++;
   // lastwin may be aucmd_win
-  win_enter(lastwin_nofloating(), false);
+  win_enter(lastwin_nofloating(NULL), false);
   autocmd_no_leave++;
   for (buf_T *buf = firstbuf; buf != NULL && open_wins < count; buf = buf->b_next) {
     // Check if this buffer needs a window
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 08d01700d6..9433957762 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -4062,7 +4062,7 @@ void compute_cmdrow(void)
   if (exmode_active || msg_scrolled != 0) {
     cmdline_row = Rows - 1;
   } else {
-    win_T *wp = lastwin_nofloating();
+    win_T *wp = lastwin_nofloating(NULL);
     cmdline_row = wp->w_winrow + wp->w_height
                   + wp->w_hsep_height + wp->w_status_height + global_stl_height();
   }
diff --git a/src/nvim/statusline.c b/src/nvim/statusline.c
index 005a52b675..82febeb6c5 100644
--- a/src/nvim/statusline.c
+++ b/src/nvim/statusline.c
@@ -440,7 +440,7 @@ void win_redr_winbar(win_T *wp)
 void redraw_ruler(void)
 {
   static int did_ruler_col = -1;
-  win_T *wp = curwin->w_status_height == 0 ? curwin : lastwin_nofloating();
+  win_T *wp = curwin->w_status_height == 0 ? curwin : lastwin_nofloating(NULL);
   bool is_stl_global = global_stl_height() > 0;
 
   // Check if ruler should be drawn, clear if it was drawn before.
diff --git a/src/nvim/window.c b/src/nvim/window.c
index bdc3234532..da4f74fc76 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -28,7 +28,6 @@
 #include "nvim/eval/window.h"
 #include "nvim/ex_cmds.h"
 #include "nvim/ex_cmds2.h"
-#include "nvim/ex_cmds_defs.h"
 #include "nvim/ex_docmd.h"
 #include "nvim/ex_eval.h"
 #include "nvim/ex_getln.h"
@@ -143,12 +142,26 @@ bool frames_locked(void)
 /// error message.  When closing window(s) and the command isn't easy to know,
 /// passing CMD_SIZE will also work.
 bool window_layout_locked(cmdidx_T cmd)
+{
+  Error err = ERROR_INIT;
+  const bool locked = window_layout_locked_err(cmd, &err);
+  if (ERROR_SET(&err)) {
+    emsg(_(err.msg));
+    api_clear_error(&err);
+  }
+  return locked;
+}
+
+/// Like `window_layout_locked`, but set `err` to the (untranslated) error message when locked.
+/// @see window_layout_locked
+bool window_layout_locked_err(cmdidx_T cmd, Error *err)
 {
   if (split_disallowed > 0 || close_disallowed > 0) {
     if (close_disallowed == 0 && cmd == CMD_tabnew) {
-      emsg(_(e_cannot_split_window_when_closing_buffer));
+      api_set_error(err, kErrorTypeException, "%s", e_cannot_split_window_when_closing_buffer);
     } else {
-      emsg(_(e_not_allowed_to_change_window_layout_in_this_autocmd));
+      api_set_error(err, kErrorTypeException, "%s",
+                    e_not_allowed_to_change_window_layout_in_this_autocmd);
     }
     return true;
   }
@@ -505,7 +518,7 @@ newwindow:
   // cursor to bottom-right window
   case 'b':
   case Ctrl_B:
-    win_goto(lastwin_nofloating());
+    win_goto(lastwin_nofloating(NULL));
     break;
 
   // cursor to last accessed (previous) window
@@ -1158,7 +1171,7 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
     oldwin = firstwin;
   } else if (flags & WSP_BOT || curwin->w_floating) {
     // can't split float, use last nonfloating window instead
-    oldwin = lastwin_nofloating();
+    oldwin = lastwin_nofloating(NULL);
   } else {
     oldwin = curwin;
   }
@@ -4800,7 +4813,7 @@ static void tabpage_check_windows(tabpage_T *old_curtab)
     if (wp->w_floating) {
       if (wp->w_config.external) {
         win_remove(wp, old_curtab);
-        win_append(lastwin_nofloating(), wp, NULL);
+        win_append(lastwin_nofloating(NULL), wp, NULL);
       } else {
         ui_comp_remove_grid(&wp->w_grid_alloc);
       }
@@ -6237,7 +6250,7 @@ static void frame_setheight(frame_T *curfrp, int height)
       if (curfrp->fr_width != Columns) {
         room_cmdline = 0;
       } else {
-        win_T *wp = lastwin_nofloating();
+        win_T *wp = lastwin_nofloating(NULL);
         room_cmdline = Rows - (int)p_ch - global_stl_height()
                        - (wp->w_winrow + wp->w_height + wp->w_hsep_height + wp->w_status_height);
         room_cmdline = MAX(room_cmdline, 0);
@@ -7051,7 +7064,7 @@ void command_height(void)
   int old_p_ch = (int)curtab->tp_ch_used;
 
   // Find bottom frame with width of screen.
-  frame_T *frp = lastwin_nofloating()->w_frame;
+  frame_T *frp = lastwin_nofloating(NULL)->w_frame;
   while (frp->fr_width != Columns && frp->fr_parent != NULL) {
     frp = frp->fr_parent;
   }
@@ -7804,9 +7817,11 @@ void win_ui_flush(bool validate)
   msg_ui_flush();
 }
 
-win_T *lastwin_nofloating(void)
+/// @return last non-floating window in `tp`, or NULL for current tabpage.
+win_T *lastwin_nofloating(tabpage_T *tp)
 {
-  win_T *res = lastwin;
+  assert(tp != curtab || !tp);
+  win_T *res = tp ? tp->tp_lastwin : lastwin;
   while (res->w_floating) {
     res = res->w_prev;
   }
diff --git a/src/nvim/winfloat.c b/src/nvim/winfloat.c
index 27d4b859d9..f222f6b17e 100644
--- a/src/nvim/winfloat.c
+++ b/src/nvim/winfloat.c
@@ -35,10 +35,9 @@
 
 #include "winfloat.c.generated.h"
 
-/// Create a new float.
+/// Creates a new float, or transforms an existing window to a float.
 ///
 /// @param wp      if NULL, allocate a new window, otherwise turn existing window into a float.
-///                It must then already belong to the current tabpage!
 /// @param last    make the window the last one in the window list.
 ///                Only used when allocating the autocommand window.
 /// @param config  must already have been validated!
@@ -46,7 +45,7 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
 {
   if (wp == NULL) {
     tabpage_T *tp = NULL;
-    win_T *tp_last = last ? lastwin : lastwin_nofloating();
+    win_T *tp_last = last ? lastwin : lastwin_nofloating(NULL);
     if (fconfig.window != 0) {
       assert(!last);
       win_T *parent_wp = find_window_by_handle(fconfig.window, err);
@@ -57,10 +56,7 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
       if (!tp) {
         return NULL;
       }
-      tp_last = tp == curtab ? lastwin : tp->tp_lastwin;
-      while (tp_last->w_floating && tp_last->w_prev) {
-        tp_last = tp_last->w_prev;
-      }
+      tp_last = lastwin_nofloating(tp == curtab ? NULL : tp);
     }
     wp = win_alloc(tp_last, false);
     win_init(wp, curwin, 0);
@@ -77,19 +73,19 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
   } else {
     assert(!last);
     assert(!wp->w_floating);
-    if (firstwin == wp && lastwin_nofloating() == wp) {
-      // last non-float
-      api_set_error(err, kErrorTypeException,
-                    "Cannot change last window into float");
-      return NULL;
-    } else if (!win_valid(wp)) {
-      api_set_error(err, kErrorTypeException,
-                    "Cannot change window from different tabpage into float");
+    tabpage_T *win_tp = win_find_tabpage(wp);
+    assert(win_tp);
+    if ((win_tp == curtab && firstwin == wp && lastwin_nofloating(NULL) == wp)
+        || (win_tp != curtab && win_tp->tp_firstwin == wp && lastwin_nofloating(win_tp) == wp)) {
+      api_set_error(err, kErrorTypeException, "Cannot change last window into float");
       return NULL;
     } else if (cmdwin_win != NULL && !cmdwin_win->w_floating) {
       // cmdwin can't become the only non-float. Check for others.
       bool other_nonfloat = false;
-      for (win_T *wp2 = firstwin; wp2 != NULL && !wp2->w_floating; wp2 = wp2->w_next) {
+      FOR_ALL_WINDOWS_IN_TAB(wp2, win_tp) {
+        if (wp2->w_floating) {
+          break;
+        }
         if (wp2 != wp && wp2 != cmdwin_win) {
           other_nonfloat = true;
           break;
@@ -100,12 +96,16 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
         return NULL;
       }
     }
+    tabpage_T *tp = win_tp == curtab ? NULL : win_tp;
     int dir;
-    winframe_remove(wp, &dir, NULL, NULL);
+    winframe_remove(wp, &dir, tp, NULL);
     XFREE_CLEAR(wp->w_frame);
-    win_comp_pos();  // recompute window positions
-    win_remove(wp, NULL);
-    win_append(lastwin_nofloating(), wp, NULL);
+    win_remove(wp, tp);
+    if (win_tp == curtab) {
+      last_status(false);  // may need to remove last status line
+      win_comp_pos();  // recompute window positions
+    }
+    win_append(lastwin_nofloating(tp), wp, tp);
   }
   wp->w_floating = true;
   wp->w_status_height = wp->w_p_stl && *wp->w_p_stl != NUL
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 62cc514058..bd7189e084 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -2561,60 +2561,88 @@ describe('API/win', function()
       eq(win, layout[2][2][2])
     end)
 
-    it('moves splits to other tabpages', function()
-      local curtab = api.nvim_get_current_tabpage()
+    it('moves windows to other tabpages', function()
+      local first_tab = api.nvim_get_current_tabpage()
+      local first_win = api.nvim_get_current_win()
       local win = api.nvim_open_win(0, false, { split = 'left' })
       command('tabnew')
-      local tabnr = api.nvim_get_current_tabpage()
-      command('tabprev') -- return to the initial tab
-
-      api.nvim_win_set_config(win, {
-        split = 'right',
-        win = api.nvim_tabpage_get_win(tabnr),
-      })
-
-      eq(tabnr, api.nvim_win_get_tabpage(win))
+      local new_tab = api.nvim_get_current_tabpage()
+      local tab2_win = api.nvim_get_current_win()
+      api.nvim_set_current_tabpage(first_tab)
+      -- move new win to new tabpage
+      api.nvim_win_set_config(win, { split = 'right', win = api.nvim_tabpage_get_win(new_tab) })
+      eq(new_tab, api.nvim_win_get_tabpage(win))
       -- we are changing the config, the current tabpage should not change
-      eq(curtab, api.nvim_get_current_tabpage())
+      eq(first_tab, api.nvim_get_current_tabpage())
 
-      command('tabnext') -- switch to the new tabpage so we can get the layout
+      api.nvim_set_current_tabpage(new_tab)
       local layout = fn.winlayout()
-
       eq({
         'row',
         {
-          { 'leaf', api.nvim_tabpage_get_win(tabnr) },
+          { 'leaf', api.nvim_tabpage_get_win(new_tab) },
           { 'leaf', win },
         },
       }, layout)
+
+      -- directly convert split into a float for a different tabpage
+      local win2 = api.nvim_open_win(0, true, { split = 'below' })
+      eq('', api.nvim_win_get_config(win2).relative)
+      api.nvim_win_set_config(
+        win2,
+        { relative = 'editor', row = 0, col = 0, width = 1, height = 1, win = first_win }
+      )
+      eq(first_tab, api.nvim_win_get_tabpage(win2))
+      eq('editor', api.nvim_win_get_config(win2).relative)
+      eq({ first_win, win2 }, api.nvim_tabpage_list_wins(first_tab))
+      eq({ tab2_win, win }, api.nvim_tabpage_list_wins(new_tab))
+
+      -- convert new win to float in new tabpage
+      api.nvim_win_set_config(win, { relative = 'editor', row = 2, col = 2, height = 2, width = 2 })
+      api.nvim_set_current_tabpage(first_tab)
+      -- move to other tabpage
+      api.nvim_win_set_config(win, { win = first_win })
+      eq(first_tab, api.nvim_win_get_tabpage(win))
+      eq({ first_win, win, win2 }, api.nvim_tabpage_list_wins(first_tab))
+      eq({ tab2_win }, api.nvim_tabpage_list_wins(new_tab))
+      -- unlike splits, negative win is invalid
+      eq('Invalid window id: -1', pcall_err(api.nvim_win_set_config, win, { win = -1 }))
+
+      -- can't convert only window in other tabpage to float
+      command('tabnew')
+      local only_win = api.nvim_get_current_win()
+      command('tabprevious')
+      eq(
+        'Cannot change last window into float',
+        pcall_err(
+          api.nvim_win_set_config,
+          only_win,
+          { relative = 'editor', width = 5, height = 5, row = 0, col = 0 }
+        )
+      )
     end)
 
     it('correctly moves curwin when moving curwin to a different tabpage', function()
-      local curtab = api.nvim_get_current_tabpage()
+      local tab1 = api.nvim_get_current_tabpage()
+      local tab1_win = api.nvim_get_current_win()
       command('tabnew')
       local tab2 = api.nvim_get_current_tabpage()
       local tab2_win = api.nvim_get_current_win()
-
-      command('tabprev') -- return to the initial tab
-
-      local neighbor = api.nvim_get_current_win()
-
+      api.nvim_set_current_tabpage(tab1) -- return to the initial tab
       -- create and enter a new split
       local win = api.nvim_open_win(0, true, {
         vertical = false,
       })
 
-      eq(curtab, api.nvim_win_get_tabpage(win))
-
-      eq({ win, neighbor }, api.nvim_tabpage_list_wins(curtab))
+      eq(tab1, api.nvim_win_get_tabpage(win))
+      eq({ win, tab1_win }, api.nvim_tabpage_list_wins(tab1))
 
       -- move the current win to a different tabpage
       api.nvim_win_set_config(win, {
         split = 'right',
         win = api.nvim_tabpage_get_win(tab2),
       })
-
-      eq(curtab, api.nvim_get_current_tabpage())
+      eq(tab1, api.nvim_get_current_tabpage())
 
       -- win should have moved to tab2
       eq(tab2, api.nvim_win_get_tabpage(win))
@@ -2622,10 +2650,18 @@ describe('API/win', function()
       eq(tab2_win, api.nvim_tabpage_get_win(tab2))
       -- win lists should be correct
       eq({ tab2_win, win }, api.nvim_tabpage_list_wins(tab2))
-      eq({ neighbor }, api.nvim_tabpage_list_wins(curtab))
-
+      eq({ tab1_win }, api.nvim_tabpage_list_wins(tab1))
       -- current win should have moved to neighboring win
-      eq(neighbor, api.nvim_tabpage_get_win(curtab))
+      eq(tab1_win, api.nvim_tabpage_get_win(tab1))
+
+      api.nvim_set_current_tabpage(tab2)
+      -- convert new win to float
+      api.nvim_win_set_config(win, { relative = 'editor', row = 2, col = 2, height = 2, width = 2 })
+      api.nvim_set_current_win(win)
+      api.nvim_win_set_config(win, { relative = 'win', win = tab1_win, row = 3, col = 3 })
+      eq(tab1, api.nvim_win_get_tabpage(win))
+      eq(tab2, api.nvim_get_current_tabpage())
+      eq({ tab1_win, win }, api.nvim_tabpage_list_wins(tab1))
     end)
 
     it('splits windows in non-current tabpage', function()
@@ -2752,7 +2788,6 @@ describe('API/win', function()
 
     it('messing with "win" or "parent" when moving "win" to other tabpage', function()
       command('split | tabnew')
-      local t2 = api.nvim_get_current_tabpage()
       local t2_win1 = api.nvim_get_current_win()
       command('split')
       local t2_win2 = api.nvim_get_current_win()
@@ -2797,23 +2832,19 @@ describe('API/win', function()
       eq('', api.nvim_win_get_config(0).relative)
       eq(cur_win, api.nvim_get_current_win())
 
-      -- Try to make "parent" floating. This should give the same error as before, but because
-      -- changing a split from another tabpage into a float isn't supported yet, check for that
-      -- error instead for now.
-      -- Use ":silent!" to avoid the one second delay from printing the error message.
+      -- Try to make "parent" floating. This should give the same error as before.
       exec(([[
-        autocmd WinLeave * ++once silent!
+        autocmd WinLeave * ++once
               \ call nvim_win_set_config(%d, #{relative:'editor', row:0, col:0, width:5, height:5})
       ]]):format(t2_win3))
       cur_win = api.nvim_get_current_win()
-      api.nvim_win_set_config(0, { win = t2_win3, split = 'left' })
-      matches(
-        'Cannot change window from different tabpage into float$',
-        api.nvim_get_vvar('errmsg')
+      eq(
+        'Floating state of windows to split changed',
+        pcall_err(api.nvim_win_set_config, 0, { win = t2_win3, split = 'left' })
       )
-      -- The error doesn't abort moving the window (or maybe it should, if that's wanted?)
-      neq(cur_win, api.nvim_get_current_win())
-      eq(t2, api.nvim_win_get_tabpage(cur_win))
+      eq('editor', api.nvim_win_get_config(t2_win3).relative)
+      eq('', api.nvim_win_get_config(0).relative)
+      eq(cur_win, api.nvim_get_current_win())
     end)
 
     it('expected autocmds when moving window to other tabpage', function()
@@ -2834,18 +2865,35 @@ describe('API/win', function()
       eq({ 'Leave', win, 'Enter', new_curwin }, eval('result'))
     end)
 
-    it('no autocmds when moving window within same tabpage', function()
+    it('no autocmds when moving window in same or other tabpage', function()
       local parent = api.nvim_get_current_win()
       exec([[
         split
-        let result = []
-        autocmd WinEnter * let result += ["Enter", win_getid()]
-        autocmd WinLeave * let result += ["Leave", win_getid()]
-        autocmd WinNew * let result += ["New", win_getid()]
+        let g:result = []
+        autocmd WinEnter * let g:result += ["Enter", win_getid()]
+        autocmd WinLeave * let g:result += ["Leave", win_getid()]
+        autocmd WinNew * let g:result += ["New", win_getid()]
       ]])
       api.nvim_win_set_config(0, { win = parent, split = 'left' })
       -- Shouldn't see any of those events, as we remain in the same window.
-      eq({}, eval('result'))
+      eq({}, eval('g:result'))
+
+      -- move float window from tab2 to tab1
+      command('tabdo only')
+      local tab1 = api.nvim_get_current_tabpage()
+      local tab1_win1 = api.nvim_get_current_win()
+      command('tabnew')
+      local fwin = api.nvim_open_win(0, false, {
+        relative = 'editor',
+        row = 2,
+        col = 2,
+        height = 2,
+        width = 2,
+      })
+      api.nvim_set_current_tabpage(tab1)
+      api.nvim_set_var('result', {})
+      api.nvim_win_set_config(fwin, { win = tab1_win1 })
+      eq({}, eval('g:result'))
     end)
 
     it('checks if splitting disallowed', function()
@@ -3120,7 +3168,6 @@ describe('API/win', function()
       )
       command('quit!')
 
-      -- Can't switch away from window before moving it to a different tabpage during textlock.
       exec(([[
         new
         call setline(1, 'foo')
@@ -3132,6 +3179,41 @@ describe('API/win', function()
         pcall_err(command, 'normal! ==')
       )
       eq(cur_win, api.nvim_get_current_win())
+      exec(([[
+        wincmd p
+        call setline(1, 'bar')
+        setlocal indentexpr=nvim_win_set_config(win_getid(winnr('#')),#{split:'left',win:%d})
+      ]]):format(t2_win))
+      neq(cur_win, api.nvim_get_current_win())
+      matches(
+        'E565: Not allowed to change text or change window$',
+        pcall_err(command, 'normal! ==')
+      )
+      -- expr_map_lock
+      exec(([[
+        nnoremap <expr> @ nvim_win_set_config(win_getid(winnr('#')),#{split:'left',win:%d})
+      ]]):format(t2_win))
+      neq(cur_win, api.nvim_get_current_win())
+      matches(
+        'E565: Not allowed to change text or change window$',
+        pcall_err(fn.feedkeys, '@', 'x')
+      )
+
+      exec(([[
+        wincmd p
+        autocmd WinNewPre * ++once call nvim_win_set_config(0, #{relative:'editor', win:%d, row:0, col:0, width:1, height:1})
+      ]]):format(t2_win))
+      matches(
+        'E1312: Not allowed to change the window layout in this autocmd$',
+        pcall_err(command, 'split')
+      )
+      eq(cur_win, api.nvim_get_current_win()) -- :split didn't enter new window due to error
+
+      exec(([[
+        autocmd WinLeave * ++once call nvim_win_set_config(0, #{relative:'editor', win:%d, row:0, col:0, width:1, height:1})
+      ]]):format(t2_win))
+      matches('Cannot move window to another tabpage whilst in use$', pcall_err(command, 'quit'))
+      eq(cur_win, api.nvim_get_current_win()) -- :quit didn't close window due to error
     end)
 
     it('updates statusline when moving bottom split', function()
@@ -3190,6 +3272,35 @@ describe('API/win', function()
       api.nvim_win_set_config(t2_cur_win, { split = 'left', win = 0 })
       eq(t2_alt_win, api.nvim_tabpage_get_win(t2))
       eq(t1, api.nvim_win_get_tabpage(t2_cur_win))
+
+      -- Very fun: move curwin between tabpages, converting from split to float, but with an autocmd
+      -- that deletes altwin after we're bumped to it, re-enters curwin, then switches to a 3rd
+      -- tabpage. tp_curwin of the window's old tabpage shouldn't be set to the freed altwin!
+      command('tablast | tab split | tabprevious | split')
+      command('autocmd WinEnter * ++once quit | let expect_alt = win_getid() | wincmd p | tabnext')
+      api.nvim_win_set_config(0, {
+        relative = 'editor',
+        win = api.nvim_tabpage_get_win(t1),
+        row = 0,
+        col = 0,
+        width = 5,
+        height = 5,
+      })
+      eq(eval('g:expect_alt'), api.nvim_tabpage_get_win(t2))
+
+      -- Same, but for float -> float.
+      command('tabprevious | split')
+      api.nvim_open_win(0, true, { relative = 'editor', row = 0, col = 0, width = 1, height = 1 })
+      command('autocmd WinEnter * ++once quit | let expect_alt = win_getid() | wincmd p | tabnext')
+      api.nvim_win_set_config(0, {
+        relative = 'editor',
+        win = api.nvim_tabpage_get_win(t1),
+        row = 0,
+        col = 0,
+        width = 5,
+        height = 5,
+      })
+      eq(eval('g:expect_alt'), api.nvim_tabpage_get_win(t2))
     end)
 
     it('set_config cannot change "noautocmd" #36409', function()
@@ -3203,6 +3314,64 @@ describe('API/win', function()
         pcall_err(api.nvim_win_set_config, win, cfg)
       )
     end)
+
+    it('removes last statusline if needed', function()
+      local screen = Screen.new(30, 9)
+      command('set laststatus=1 | botright split')
+      screen:expect([[
+                                      |
+        {1:~                             }|*2
+        {2:[No Name]                     }|
+        ^                              |
+        {1:~                             }|*2
+        {3:[No Name]                     }|
+                                      |
+      ]])
+      api.nvim_win_set_config(0, { relative = 'editor', row = 0, col = 0, width = 4, height = 4 })
+      screen:expect([[
+        {4:^    }                          |
+        {11:~   }{1:                          }|*3
+        {1:~                             }|*4
+                                      |
+      ]])
+      command('quit | set laststatus=2 | botright split')
+      screen:expect([[
+                                      |
+        {1:~                             }|*2
+        {2:[No Name]                     }|
+        ^                              |
+        {1:~                             }|*2
+        {3:[No Name]                     }|
+                                      |
+      ]])
+      api.nvim_win_set_config(0, { relative = 'editor', row = 1, col = 5, width = 4, height = 4 })
+      screen:expect([[
+                                      |
+        {1:~    }{4:^    }{1:                     }|
+        {1:~    }{11:~   }{1:                     }|*3
+        {1:~                             }|*2
+        {2:[No Name]                     }|
+                                      |
+      ]])
+    end)
+
+    it('can convert external window to non-external', function()
+      Screen.new(20, 7, { ext_multigrid = true }) -- multigrid needed for external windows
+      api.nvim_open_win(0, true, { external = true, width = 5, height = 5 })
+      eq(true, api.nvim_win_get_config(0).external)
+      api.nvim_win_set_config(0, { split = 'below', win = fn.win_getid(1) })
+      eq(false, api.nvim_win_get_config(0).external)
+
+      api.nvim_win_set_config(0, { external = true, width = 5, height = 5 })
+      eq(true, api.nvim_win_get_config(0).external)
+      api.nvim_win_set_config(0, { relative = 'editor', row = 3, col = 3 })
+      eq(false, api.nvim_win_get_config(0).external)
+
+      api.nvim_win_set_config(0, { external = true, width = 5, height = 5 })
+      eq(true, api.nvim_win_get_config(0).external)
+      api.nvim_win_set_config(0, { external = false })
+      eq(false, api.nvim_win_get_config(0).external)
+    end)
   end)
 
   describe('get_config', function()
@@ -3542,20 +3711,25 @@ describe('API/win', function()
     end)
 
     it('cannot move autocmd window between tabpages', function()
-      local win_type, split_ok, err = exec_lua(function()
+      local win_type, split_ok, split_err, float_ok, float_err = exec_lua(function()
         local other_tp_win = vim.api.nvim_get_current_win()
         vim.cmd.tabnew()
 
-        local win_type, split_ok, err
+        local win_type, split_ok, split_err, float_ok, float_err
         vim.api.nvim_buf_call(vim.api.nvim_create_buf(true, true), function()
           win_type = vim.fn.win_gettype()
-          split_ok, err =
+
+          split_ok, split_err =
             pcall(vim.api.nvim_win_set_config, 0, { win = other_tp_win, split = 'right' })
+
+          float_ok, float_err = pcall(vim.api.nvim_win_set_config, 0, { win = other_tp_win })
         end)
-        return win_type, split_ok, err
+        return win_type, split_ok, split_err, float_ok, float_err
       end)
+
       eq('autocmd', win_type)
-      eq({ false, 'Cannot move autocmd window to another tabpage' }, { split_ok, err })
+      eq({ false, 'Cannot move autocmd window to another tabpage' }, { split_ok, split_err })
+      eq({ false, 'Cannot move autocmd window to another tabpage' }, { float_ok, float_err })
     end)
 
     it('cannot move cmdwin between tabpages', function()
@@ -3656,5 +3830,35 @@ describe('API/win', function()
       api.nvim_open_win(0, true, { split = 'below', style = 'minimal' })
       command('quit')
     end)
+
+    it('preserve current floating window when moving fails', function()
+      local buf = api.nvim_create_buf(false, true)
+      local float_win = api.nvim_open_win(buf, true, {
+        relative = 'editor',
+        row = 1,
+        col = 1,
+        width = 10,
+        height = 5,
+      })
+      command('tabnew')
+      local tab2_win = api.nvim_get_current_win()
+      command('tabprev')
+      api.nvim_set_current_win(float_win)
+      command('autocmd WinLeave * ++once call nvim_win_close(' .. tab2_win .. ', v:true)')
+      eq(
+        'Target windows were closed',
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab2_win })
+      )
+      eq(float_win, api.nvim_get_current_win())
+
+      command('tabnew')
+      local tab3_win = api.nvim_get_current_win()
+      command('tabprev | autocmd WinEnter * ++once wincmd p')
+      eq(
+        ('Failed to switch away from window %d'):format(float_win),
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab3_win })
+      )
+      eq(float_win, api.nvim_get_current_win())
+    end)
   end)
 end)
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 15b7004f7e..c9453e2b32 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -3782,10 +3782,7 @@ describe('float window', function()
     it('API has proper error messages', function()
       local buf = api.nvim_create_buf(false, false)
       eq("Invalid key: 'bork'", pcall_err(api.nvim_open_win, buf, false, { width = 20, height = 2, bork = true }))
-      eq(
-        "'win' key is only valid with relative='win' and relative=''",
-        pcall_err(api.nvim_open_win, buf, false, { width = 20, height = 2, relative = 'editor', row = 0, col = 0, win = 0 })
-      )
+      eq("Must specify 'relative' or 'external' when creating a float", pcall_err(api.nvim_open_win, buf, false, { win = 0 }))
       eq(
         "floating windows cannot have 'vertical'",
         pcall_err(api.nvim_open_win, buf, false, { width = 20, height = 2, relative = 'editor', row = 0, col = 0, vertical = true })
@@ -3824,6 +3821,18 @@ describe('float window', function()
       )
       eq("Must specify 'width'", pcall_err(api.nvim_open_win, buf, false, { relative = 'editor', row = 0, col = 0 }))
       eq("Must specify 'height'", pcall_err(api.nvim_open_win, buf, false, { relative = 'editor', row = 0, col = 0, width = 2 }))
+
+      if multigrid then
+        eq(
+          "external window cannot have 'win'",
+          pcall_err(api.nvim_open_win, buf, false, { external = true, win = 0, width = 10, height = 10 })
+        )
+        api.nvim_open_win(buf, true, { external = true, width = 10, height = 10 })
+        eq("external window cannot have 'win'", pcall_err(api.nvim_win_set_config, 0, { win = 0 }))
+        -- OK to include "win" if external window is also reconfigured to a normal float.
+        api.nvim_win_set_config(0, { relative = 'editor', win = 0, row = 0, col = 0, width = 5, height = 5 })
+        eq('editor', api.nvim_win_get_config(0).relative)
+      end
     end)
 
     it('can be placed relative window or cursor', function()
@@ -7758,12 +7767,11 @@ describe('float window', function()
           screen:expect {
             grid = [[
           ## grid 1
-            [2:----------------------------------------]|*5
-            {5:[No Name] [+]                           }|
+            [2:----------------------------------------]|*6
             [3:----------------------------------------]|
           ## grid 2
             x                                       |
-            {0:~                                       }|*4
+            {0:~                                       }|*5
           ## grid 3
                                                     |
           ## grid 4
@@ -11191,6 +11199,19 @@ describe('float window', function()
       local winid = api.nvim_open_win(buf, false, config)
       api.nvim_set_current_win(winid)
       eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, winid, config))
+      eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, winid, { win = winid }))
+      -- Don't assume win=0 if no win given for existing relative=win float; so no error.
+      api.nvim_win_set_config(winid, { width = 7 })
+      eq(7, api.nvim_win_get_config(winid).width)
+      -- Don't expect the error when configuring to something other than relative=win, as win=self
+      -- is fine in those cases. (though maybe pointless) Other errors might be expected, though.
+      eq('Cannot split a floating window', pcall_err(api.nvim_win_set_config, winid, { split = 'above', win = winid }))
+      eq('win', api.nvim_win_get_config(winid).relative)
+      api.nvim_win_set_config(winid, { relative = 'editor', win = winid, row = 3, col = 3 })
+      eq('editor', api.nvim_win_get_config(winid).relative)
+      -- An error when configuring split into relative=win float.
+      command('split')
+      eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, 0, config))
     end)
 
     it('bufpos out of range', function()
@@ -11869,6 +11890,485 @@ describe('float window', function()
         ]])
       end
     end)
+
+    it('redrawn after moving tabpages via nvim_win_set_config()', function()
+      local tab1_win = api.nvim_get_current_win()
+      fn.setline(1, 'hello')
+      command('tab split')
+      local tab2_win = api.nvim_get_current_win()
+      -- Schedule an UPD_NOT_VALID redraw, but in one event move the float out of curtab before it's
+      -- handled. Do not flush before then.
+      local float = exec_lua(function()
+        local float = vim.api.nvim_open_win(0, true, { relative = 'editor', width = 10, height = 5, row = 1, col = 1 })
+        vim.api.nvim__redraw({ valid = false, flush = false })
+        vim.api.nvim_win_set_config(float, { win = tab1_win })
+        return float
+      end)
+
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ [No Name] }{3: + [No Name] }{5:            }{9:X}|
+            [4:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+            hello                                   |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 4
+            ^hello                                   |
+            {0:~                                       }|*4
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ [No Name] }{3: + [No Name] }{5:            }{9:X}|
+          ^hello                                   |
+          {0:~                                       }|*4
+                                                  |
+        ]])
+      end
+
+      -- Importantly, want tabline redrawn and float's hl attribs to be correct here.
+      api.nvim_win_set_config(float, { win = 0 })
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: + [No Name] }{3: }{11:2}{3:+ [No Name] }{5:            }{9:X}|
+            [4:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+            hello                                   |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 4
+            ^hello                                   |
+            {0:~                                       }|*4
+          ## grid 5
+            {1:hello     }|
+            {2:~         }|*4
+          ]],
+          float_pos = {
+            [5] = { 1002, 'NW', 1, 1, 1, true, 50, 1, 1, 1 },
+          },
+        })
+      else
+        screen:expect([[
+          {9: + [No Name] }{3: }{11:2}{3:+ [No Name] }{5:            }{9:X}|
+          ^h{1:hello     }                             |
+          {0:~}{2:~         }{0:                             }|*4
+                                                  |
+        ]])
+      end
+
+      -- Autocommand runs within the first tabpage, but won't refresh grids when switching to it due
+      -- to switch_win_noblock having no_display set.
+      command(
+        ('autocmd OptionSet rightleft ++once call nvim_win_set_config(%d, #{relative: "win", win: %d, row: 0, col: 0})'):format(
+          float,
+          tab1_win
+        )
+      )
+      api.nvim_set_option_value('rightleft', true, { win = tab1_win })
+      -- Stale grids should not have caused issues with removing the float's grid.
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ [No Name] }{3: + [No Name] }{5:            }{9:X}|
+            [4:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+            hello                                   |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 4
+            ^hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ [No Name] }{3: + [No Name] }{5:            }{9:X}|
+          ^hello                                   |
+          {0:~                                       }|*4
+                                                  |
+        ]])
+      end
+
+      command('tabfirst')
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {3: }{11:2}{3:+ [No Name] }{9: + [No Name] }{5:            }{9:X}|
+            [2:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2
+                                               olle^h|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5
+            {1:hello     }|
+            {2:~         }|*4
+          ]],
+          float_pos = {
+            [5] = { 1002, 'NW', 2, 0, 0, true, 50, 1, 1, 0 },
+          },
+        })
+      else
+        screen:expect([[
+          {3: }{11:2}{3:+ [No Name] }{9: + [No Name] }{5:            }{9:X}|
+          {1:hello     }                         olle^h|
+          {2:~         }{0:                             ~}|*4
+                                                  |
+        ]])
+      end
+
+      -- Check tablines are redrawn even when moving floats between two non-current tabpages.
+      command('tabnew')
+      local tab3_win = api.nvim_get_current_win()
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: + [No Name] }{5:  }{9:X}|
+            [6:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6
+                                                   ^ |
+            {0:                                       ~}|*4
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: + [No Name] }{5:  }{9:X}|
+                                                 ^ |
+          {0:                                       ~}|*4
+                                                  |
+        ]])
+      end
+
+      api.nvim_win_set_config(float, { win = tab2_win })
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: + [No Name] }{3: [No Name] }{9: }{10:2}{9:+ No Name] }{5:  }{9:X}|
+            [6:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6
+                                                   ^ |
+            {0:                                       ~}|*4
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: + [No Name] }{3: [No Name] }{9: }{10:2}{9:+ No Name] }{5:  }{9:X}|
+                                                 ^ |
+          {0:                                       ~}|*4
+                                                  |
+        ]])
+      end
+
+      -- Try converting a split to a float, then moving it to another tabpage in one call.
+      command('set norightleft | new')
+      fn.setline(1, 'floaty mcfloatface')
+      api.nvim_win_set_config(0, { relative = 'editor', win = tab1_win, row = 3, col = 3, width = 15, height = 5 })
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: }{10:2}{9:+ No Name] }{5:  }{9:X}|
+            [6:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6
+            ^                                        |
+            {0:~                                       }|*4
+          ## grid 7 (hidden)
+            floaty mcfloatface                      |
+            {0:~                                       }|
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: }{10:2}{9:+ No Name] }{5:  }{9:X}|
+          ^                                        |
+          {0:~                                       }|*4
+                                                  |
+        ]])
+      end
+
+      command('tabfirst')
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {3: }{11:2}{3:+ No Name] }{9: [No Name]  }{10:2}{9:+ No Name] }{5:  }{9:X}|
+            [2:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2
+                                               olle^h|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6 (hidden)
+                                                    |
+            {0:~                                       }|*4
+          ## grid 7
+            {1:floaty mcfloatf}|
+            {1:ace            }|
+            {2:~              }|*3
+          ]],
+          float_pos = {
+            [7] = { 1004, 'NW', 1, 3, 3, true, 50, 1, 1, 3 },
+          },
+        })
+      else
+        screen:expect([[
+          {3: }{11:2}{3:+ No Name] }{9: [No Name]  }{10:2}{9:+ No Name] }{5:  }{9:X}|
+             {1:floaty mcfloatf}                 olle^h|
+          {0:   }{1:ace            }{0:                     ~}|
+          {0:   }{2:~              }{0:                     ~}|*3
+                                                  |
+        ]])
+      end
+
+      -- Works when doing the same between two non-current tabpages.
+      local float2 = api.nvim_open_win(0, false, { split = 'below', win = tab3_win })
+      api.nvim_win_set_config(float2, { relative = 'win', win = tab2_win, row = 2, col = 7, width = 4, height = 4, border = 'single' })
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {3: }{11:2}{3:+ No Name] }{9: [No Name]  }{10:3}{9:+ No Name] }{5:  }{9:X}|
+            [2:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2
+                                               olle^h|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6 (hidden)
+                                                    |
+            {0:~                                       }|*4
+          ## grid 7
+            {1:floaty mcfloatf}|
+            {1:ace            }|
+            {2:~              }|*3
+          ]],
+          float_pos = {
+            [7] = { 1004, 'NW', 1, 3, 3, true, 50, 1, 1, 3 },
+          },
+        })
+      else
+        screen:expect([[
+          {3: }{11:2}{3:+ No Name] }{9: [No Name]  }{10:3}{9:+ No Name] }{5:  }{9:X}|
+             {1:floaty mcfloatf}                 olle^h|
+          {0:   }{1:ace            }{0:                     ~}|
+          {0:   }{2:~              }{0:                     ~}|*3
+                                                  |
+        ]])
+      end
+
+      command('tabnext')
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: }{10:3}{9:+ No Name] }{5:  }{9:X}|
+            [6:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4 (hidden)
+            hello                                   |
+            {0:~                                       }|*4
+          ## grid 5 (hidden)
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6
+            ^                                        |
+            {0:~                                       }|*4
+          ## grid 7 (hidden)
+            {1:floaty mcfloatf}|
+            {1:ace            }|
+            {2:~              }|*3
+          ]],
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ No Name] }{3: [No Name] }{9: }{10:3}{9:+ No Name] }{5:  }{9:X}|
+          ^                                        |
+          {0:~                                       }|*4
+                                                  |
+        ]])
+      end
+
+      command('tabnext')
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ No Name]  [No Name] }{3: }{11:3}{3:+ No Name] }{5:  }{9:X}|
+            [4:----------------------------------------]|*5
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4
+            ^hello                                   |
+            {0:~                                       }|*4
+          ## grid 5
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6 (hidden)
+                                                    |
+            {0:~                                       }|*4
+          ## grid 7 (hidden)
+            {1:floaty mcfloatf}|
+            {1:ace            }|
+            {2:~              }|*3
+          ## grid 8
+            {5:┌────┐}|
+            {5:│}{1:lleh}{5:│}|
+            {5:│}{1:   o}{5:│}|
+            {5:│}{2:   ~}{5:│}|*2
+            {5:└────┘}|
+          ]],
+          float_pos = {
+            [5] = { 1002, 'NW', 4, 0, 0, true, 50, 1, 1, 0 },
+            [8] = { 1005, 'NW', 4, 2, 7, true, 50, 2, 0, 7 },
+          },
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ No }{5:┌────┐}{9: [No Name] }{3: }{11:3}{3:+ No Name] }{5:  }{9:X}|
+          {1:^hello  }{5:│}{1:lleh}{5:│}                           |
+          {2:~      }{5:│}{1:   o}{5:│}{0:                           }|
+          {2:~      }{5:│}{2:   ~}{5:│}{0:                           }|*2
+          {2:~      }{5:└────┘}{0:                           }|
+                                                  |
+        ]])
+      end
+
+      -- Used relative=win on two floats relative to this window.
+      -- Split it to the right to ensure both follow.
+      command('topleft vsplit')
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            {9: }{10:2}{9:+ No Name]  [No Name] }{3: }{11:4}{3:+ No Name] }{5:  }{9:X}|
+            [9:--------------------]{5:│}[4:-------------------]|*4
+            {4:[No Name] [+]        }{5:[No Name] [+]      }|
+            [3:----------------------------------------]|
+          ## grid 2 (hidden)
+                                               olleh|
+            {0:                                       ~}|*4
+          ## grid 3
+                                                    |
+          ## grid 4
+            hello              |
+            {0:~                  }|*3
+          ## grid 5
+            {1:hello     }|
+            {2:~         }|*4
+          ## grid 6 (hidden)
+                                                    |
+            {0:~                                       }|*4
+          ## grid 7 (hidden)
+            {1:floaty mcfloatf}|
+            {1:ace            }|
+            {2:~              }|*3
+          ## grid 8
+            {5:┌────┐}|
+            {5:│}{1:lleh}{5:│}|
+            {5:│}{1:   o}{5:│}|
+            {5:│}{2:   ~}{5:│}|*2
+            {5:└────┘}|
+          ## grid 9
+            ^hello               |
+            {0:~                   }|*3
+          ]],
+          float_pos = {
+            [5] = { 1002, 'NW', 4, 0, 0, true, 50, 1, 1, 21 },
+            [8] = { 1005, 'NW', 4, 2, 7, true, 50, 2, 0, 28 },
+          },
+        })
+      else
+        screen:expect([[
+          {9: }{10:2}{9:+ No Name]  [No Name] }{3: }{11:4}{3:+ }{5:┌────┐}{3:e] }{5:  }{9:X}|
+          ^hello               {5:│}{1:hello  }{5:│}{1:lleh}{5:│}      |
+          {0:~                   }{5:│}{2:~      }{5:│}{1:   o}{5:│}{0:      }|
+          {0:~                   }{5:│}{2:~      }{5:│}{2:   ~}{5:│}{0:      }|*2
+          {4:[No Name] [+]        }{2:~      }{5:└────┘      }|
+                                                  |
+        ]])
+      end
+    end)
   end
 
   describe('with ext_multigrid and actual mouse grid', function()
diff --git a/test/functional/ui/messages_spec.lua b/test/functional/ui/messages_spec.lua
index 4159563fcf..223088b3e8 100644
--- a/test/functional/ui/messages_spec.lua
+++ b/test/functional/ui/messages_spec.lua
@@ -2346,8 +2346,7 @@ describe('ui/ext_messages', function()
     screen:expect([[
                                                                                       |
       {1:~    }{4:^     }{1:                                                                      }|
-      {1:~                                                                               }|*21
-      {2:[No Name]                                                                       }|
+      {1:~                                                                               }|*22
     ]])
   end)
 
diff --git a/test/functional/ui/screen.lua b/test/functional/ui/screen.lua
index d9501b8400..d60992974a 100644
--- a/test/functional/ui/screen.lua
+++ b/test/functional/ui/screen.lua
@@ -1795,9 +1795,14 @@ local function fmt_ext_state(name, state)
   elseif name == 'float_pos' then
     local str = '{\n'
     for k, v in pairs(state) do
-      str = str .. '  [' .. k .. '] = {' .. v[1]
-      for i = 2, #v do
-        str = str .. ', ' .. inspect(v[i])
+      str = str .. '  [' .. k .. '] = {'
+      if v.external then
+        str = str .. ' external = true '
+      else
+        str = str .. v[1]
+        for i = 2, #v do
+          str = str .. ', ' .. inspect(v[i])
+        end
       end
       str = str .. '};\n'
     end