fix(float): changing zindex does not restack drawn float #42571

Problem:
nvim_win_set_config() with only a new zindex does not change which
float is drawn on top.

Solution:
Recompose the grid area after its layer is reordered.

src/nvim/window.c

@@ -957,6 +957,9 @@ void ui_ext_win_position(win_T *wp, bool validate)
     if (!c.hide) {
       ui_comp_put_grid(&wp->w_grid_alloc, comp_row, comp_col,
                        wp->w_height_outer, wp->w_width_outer, valid, false);
+      if (resort) {
+        ui_comp_compose_grid(&wp->w_grid_alloc);
+      }
       if (ui_has(kUIMultigrid)) {
         String anchor = cstr_as_string(float_anchor_str[c.anchor]);
         ui_call_win_float_pos(wp->w_grid_alloc.handle, wp->handle, anchor,

test/functional/ui/float_spec.lua

@@ -10016,6 +10016,50 @@ describe('float window', function()
         }
       end
 
+      -- changing only zindex restacks the float #42556
+      api.nvim_win_set_config(win1, { zindex = 60 })
+      if multigrid then
+        screen:expect {
+          grid = [[
+        ## grid 1
+          [2:----------------------------------------]|*6
+          [3:----------------------------------------]|
+        ## grid 2
+          ^                                        |
+          {0:~                                       }|*5
+        ## grid 3
+                                                  |
+        ## grid 4
+          {7:                    }|
+          {7:~                   }|*2
+        ## grid 5
+          {17:                    }|
+          {17:~                   }|*2
+        ## grid 6
+          {8:                    }|
+          {8:~                   }|*2
+        ]],
+          float_pos = {
+            [4] = { 1001, 'NW', 1, 1, 5, true, 60, 3, 1, 5 },
+            [5] = { 1002, 'NW', 1, 2, 6, true, 50, 2, 2, 6 },
+            [6] = { 1003, 'NW', 1, 3, 7, true, 40, 1, 3, 7 },
+          },
+        }
+      else
+        screen:expect {
+          grid = [[
+          ^                                        |
+          {0:~    }{7:                    }{0:               }|
+          {0:~    }{7:~                   }{17: }{0:              }|
+          {0:~    }{7:~                   }{17: }{8: }{0:             }|
+          {0:~     }{17:~                   }{8: }{0:             }|
+          {0:~      }{8:~                   }{0:             }|
+                                                  |
+        ]],
+        }
+      end
+      api.nvim_win_set_config(win1, { zindex = 30 })
+
       --
       -- Check that floats are positioned correctly after changing the zindexes.
       --