feat(api): nvim_open_win() relative to tabline and laststatus #32006

Problem:  Anchoring a floating window to the tabline and laststatus is
          cumbersome; requiring autocommands and looping over all
          windows/tabpages.
Solution: Add new "tabline" and "laststatus" options to the `relative`
          field of nvim_open_win() to place a window relative to.

src/nvim/api/win_config.c

@@ -101,10 +101,12 @@
 /// @param config Map defining the window configuration. Keys:
 ///   - relative: Sets the window layout to "floating", placed at (row,col)
 ///                 coordinates relative to:
-///      - "editor" The global editor grid
-///      - "win"    Window given by the `win` field, or current window.
-///      - "cursor" Cursor position in current window.
-///      - "mouse"  Mouse position
+///      - "cursor"     Cursor position in current window.
+///      - "editor"     The global editor grid.
+///      - "laststatus" 'laststatus' if present, or last row.
+///      - "mouse"      Mouse position.
+///      - "tabline"    Tabline if present, or first row.
+///      - "win"        Window given by the `win` field, or current window.
 ///   - win: |window-ID| window to split, or relative window when creating a
 ///      float (relative="win").
 ///   - anchor: Decides which corner of the float to place at (row,col):
@@ -699,7 +701,9 @@ Dict(win_config) nvim_win_get_config(Window window, Arena *arena, Error *err)
   FUNC_API_SINCE(6)
 {
   /// Keep in sync with FloatRelative in buffer_defs.h
-  static const char *const float_relative_str[] = { "editor", "win", "cursor", "mouse" };
+  static const char *const float_relative_str[] = {
+    "editor", "win", "cursor", "mouse", "tabline", "laststatus"
+  };
 
   /// Keep in sync with WinSplit in buffer_defs.h
   static const char *const win_split_str[] = { "left", "right", "above", "below" };
@@ -805,6 +809,10 @@ static bool parse_float_relative(String relative, FloatRelative *out)
     *out = kFloatRelativeCursor;
   } else if (striequal(str, "mouse")) {
     *out = kFloatRelativeMouse;
+  } else if (striequal(str, "tabline")) {
+    *out = kFloatRelativeTabline;
+  } else if (striequal(str, "laststatus")) {
+    *out = kFloatRelativeLaststatus;
   } else {
     return false;
   }

src/nvim/buffer_defs.h

@@ -900,6 +900,8 @@ typedef enum {
   kFloatRelativeWindow = 1,
   kFloatRelativeCursor = 2,
   kFloatRelativeMouse = 3,
+  kFloatRelativeTabline = 4,
+  kFloatRelativeLaststatus = 5,
 } FloatRelative;
 
 /// Keep in sync with win_split_str[] in nvim_win_get_config() (api/win_config.c)

src/nvim/window.c

@@ -835,6 +835,10 @@ void ui_ext_win_position(win_T *wp, bool validate)
           col += tcol - 1;
         }
       }
+    } else if (c.relative == kFloatRelativeLaststatus) {
+      row += Rows - (int)p_ch - last_stl_height(false);
+    } else if (c.relative == kFloatRelativeTabline) {
+      row += tabline_height();
     }
 
     bool resort = wp->w_grid_alloc.comp_index != 0
@@ -1066,6 +1070,7 @@ win_T *win_split_ins(int size, int flags, win_T *new_wp, int dir, frame_T *to_fl
       return NULL;
     }
     need_status = STATUS_HEIGHT;
+    win_float_anchor_laststatus();
   }
 
   bool do_equal = false;
@@ -6803,6 +6808,7 @@ void last_status(bool morewin)
 {
   // Don't make a difference between horizontal or vertical split.
   last_status_rec(topframe, last_stl_height(morewin) > 0, global_stl_height() > 0);
+  win_float_anchor_laststatus();
 }
 
 // Remove status line from window, replacing it with a horizontal separator if needed.

src/nvim/winfloat.c

@@ -307,6 +307,15 @@ void win_check_anchored_floats(win_T *win)
   }
 }
 
+void win_float_anchor_laststatus(void)
+{
+  FOR_ALL_WINDOWS_IN_TAB(win, curtab) {
+    if (win->w_config.relative == kFloatRelativeLaststatus) {
+      win->w_pos_changed = true;
+    }
+  }
+}
+
 void win_reconfig_floats(void)
 {
   for (win_T *wp = lastwin; wp && wp->w_floating; wp = wp->w_prev) {