fix(compositor): clear old position with last known dimensions #38229

Problem:  When reconfiguring a float reallocates the grid before the old
          area is cleared, artifacts are left on the screen.
Solution: Use the last known compositor dimensions of a grid when
          clearing the area covered by the old position.

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/ui_compositor.c

@@ -143,8 +143,6 @@ bool ui_comp_put_grid(ScreenGrid *grid, int row, int col, int height, int width,
   bool moved;
   grid->pending_comp_index_update = true;
 
-  grid->comp_height = height;
-  grid->comp_width = width;
   if (grid->comp_index != 0) {
     moved = (row != grid->comp_row) || (col != grid->comp_col);
     if (ui_comp_should_draw()) {
@@ -153,19 +151,19 @@ bool ui_comp_put_grid(ScreenGrid *grid, int row, int col, int height, int width,
       // use it.
       grid->comp_disabled = true;
       compose_area(grid->comp_row, row,
-                   grid->comp_col, grid->comp_col + grid->cols);
+                   grid->comp_col, grid->comp_col + grid->comp_width);
       if (grid->comp_col < col) {
         compose_area(MAX(row, grid->comp_row),
-                     MIN(row + height, grid->comp_row + grid->rows),
+                     MIN(row + height, grid->comp_row + grid->comp_height),
                      grid->comp_col, col);
       }
-      if (col + width < grid->comp_col + grid->cols) {
+      if (col + width < grid->comp_col + grid->comp_width) {
         compose_area(MAX(row, grid->comp_row),
-                     MIN(row + height, grid->comp_row + grid->rows),
+                     MIN(row + height, grid->comp_row + grid->comp_height),
-                     col + width, grid->comp_col + grid->cols);
+                     col + width, grid->comp_col + grid->comp_width);
       }
-      compose_area(row + height, grid->comp_row + grid->rows,
+      compose_area(row + height, grid->comp_row + grid->comp_height,
-                   grid->comp_col, grid->comp_col + grid->cols);
+                   grid->comp_col, grid->comp_col + grid->comp_width);
       grid->comp_disabled = false;
     }
     grid->comp_row = row;
@@ -204,6 +202,9 @@ bool ui_comp_put_grid(ScreenGrid *grid, int row, int col, int height, int width,
     grid->comp_index = insert_at;
     grid->pending_comp_index_update = true;
   }
+
+  grid->comp_height = height;
+  grid->comp_width = width;
   if (moved && valid && ui_comp_should_draw()) {
     compose_area(grid->comp_row, grid->comp_row + grid->rows,
                  grid->comp_col, grid->comp_col + grid->cols);

test/functional/ui/float_spec.lua

@@ -1126,6 +1126,30 @@ describe('float window', function()
     neq(tp1, api.nvim_win_get_tabpage(w6))
   end)
 
+  it('compositor clears old position when configuring reallocates grid #38143', function()
+    local screen = Screen.new()
+    local w1 = api.nvim_open_win(0, true, { relative = 'editor', border = 'single', row = 0, col = 0, width = 5, height = 5 })
+    screen:expect([[
+      {2:┌─────┐}                                              |
+      {2:│}{4:^     }{2:│}{1:                                              }|
+      {2:│}{11:~    }{2:│}{1:                                              }|*4
+      {2:└─────┘}{1:                                              }|
+      {1:~                                                    }|*6
+                                                           |
+    ]])
+    api.nvim_win_set_config(w1, { relative = 'cursor', row = 1, col = 1, height = 4, width = 4 })
+    screen:expect([[
+                                                           |
+      {1:~                                                    }|
+      {1:~ }{2:┌────┐}{1:                                             }|
+      {1:~ }{2:│}{4:^    }{2:│}{1:                                             }|
+      {1:~ }{2:│}{11:~   }{2:│}{1:                                             }|*3
+      {1:~ }{2:└────┘}{1:                                             }|
+      {1:~                                                    }|*5
+                                                           |
+    ]])
+  end)
+
   local function with_ext_multigrid(multigrid, send_mouse_grid)
     local screen, attrs
     before_each(function()