fix(api): win_set_config error message for splitting from a float #35650

Problem: after #34287 nvim_win_set_config no longer errors when attempting to
split from a floating window only if "win" is 0.

Solution: fix the regression, reduce the scope of "parent" and similar checks to
only where it's currently used and add test coverage for the errors.

src/nvim/api/win_config.c

@@ -415,25 +415,6 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
   if (!parse_win_config(win, config, &fconfig, !was_split || to_split, err)) {
     return;
   }
-  win_T *parent = config->win == 0 ? curwin : NULL;
-  tabpage_T *parent_tp = config->win == 0 ? curtab : NULL;
-
-  if (config->win > 0) {
-    parent = find_window_by_handle(fconfig.window, err);
-    if (!parent) {
-      return;
-    } else if (to_split && parent->w_floating) {
-      api_set_error(err, kErrorTypeException, "Cannot split a floating window");
-      return;
-    }
-    parent_tp = win_find_tabpage(parent);
-
-    // Prevent autocmd window from being moved into another tabpage
-    if (is_aucmd_win(win) && win_tp != parent_tp) {
-      api_set_error(err, kErrorTypeException, "Cannot move autocmd win to another tabpage");
-      return;
-    }
-  }
 
   if (was_split && !to_split) {
     if (!win_new_float(win, false, fconfig, err)) {
@@ -441,6 +422,34 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     }
     redraw_later(win, UPD_NOT_VALID);
   } else if (to_split) {
+    win_T *parent = NULL;
+    tabpage_T *parent_tp = NULL;
+    if (config->win == 0) {
+      parent = curwin;
+      parent_tp = curtab;
+    } else if (config->win > 0) {
+      parent = find_window_by_handle(fconfig.window, err);
+      if (!parent) {
+        return;
+      }
+      parent_tp = win_find_tabpage(parent);
+    }
+    if (parent) {
+      if (parent->w_floating) {
+        api_set_error(err, kErrorTypeException, "Cannot split a floating window");
+        return;
+      }
+      if (is_aucmd_win(win) && win_tp != parent_tp) {
+        api_set_error(err, kErrorTypeException, "Cannot move autocmd window to another tabpage");
+        return;
+      }
+      // 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;
+      }
+    }
+
     WinSplit old_split = win_split_dir(win);
     if (has_vertical && !has_split) {
       if (config->vertical) {
@@ -468,11 +477,6 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     if (!check_split_disallowed_err(win, err)) {
       return;  // error already set
     }
-    // Can't move the cmdwin or its old curwin to a different tabpage.
-    if ((win == cmdwin_win || win == cmdwin_old_curwin) && parent && parent_tp != win_tp) {
-      api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
-      return;
-    }
 
     bool to_split_ok = false;
     // If we are moving curwin to another tabpage, switch windows *before* we remove it from the

test/functional/api/window_spec.lua

@@ -3325,5 +3325,58 @@ describe('API/win', function()
         eq({ { 'OLD_FOOTER' } }, api.nvim_win_get_config(win).footer)
       end)
     end)
+
+    it('cannot split from a float', function()
+      local win = api.nvim_get_current_win()
+      local float_win = api.nvim_open_win(0, true, {
+        relative = 'editor',
+        width = 10,
+        height = 10,
+        row = 10,
+        col = 10,
+      })
+      eq(
+        'Cannot split a floating window',
+        pcall_err(api.nvim_win_set_config, win, { win = float_win, split = 'right' })
+      )
+      eq(
+        'Cannot split a floating window',
+        pcall_err(api.nvim_win_set_config, win, { win = 0, split = 'right' })
+      )
+    end)
+
+    it('cannot move autocmd window between tabpages', function()
+      local win_type, split_ok, err = exec_lua(function()
+        local other_tp_win = vim.api.nvim_get_current_win()
+        vim.cmd.tabnew()
+
+        local win_type, split_ok, err
+        vim.api.nvim_buf_call(vim.api.nvim_create_buf(true, true), function()
+          win_type = vim.fn.win_gettype()
+          split_ok, err =
+            pcall(vim.api.nvim_win_set_config, 0, { win = other_tp_win, split = 'right' })
+        end)
+        return win_type, split_ok, err
+      end)
+      eq('autocmd', win_type)
+      eq({ false, 'Cannot move autocmd window to another tabpage' }, { split_ok, err })
+    end)
+
+    it('cannot move cmdwin between tabpages', function()
+      local other_tp_win = api.nvim_get_current_win()
+      command('tabnew')
+      local old_curwin = api.nvim_get_current_win()
+      feed('q:')
+      eq('command', fn.win_gettype())
+      eq(
+        'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+        pcall_err(api.nvim_win_set_config, 0, { win = other_tp_win, split = 'right' })
+      )
+      -- Shouldn't move the old curwin from before we entered the cmdwin either.
+      eq(
+        'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+        pcall_err(api.nvim_win_set_config, old_curwin, { win = other_tp_win, split = 'right' })
+      )
+    end)
   end)
 end)