fix(win_close): remove float grid after closing buffer

It is not safe to remove the float grid when autocommands can still be
triggered, as autocommands may use the float grid.

(cherry picked from commit 9a5e80af56)

test/functional/ui/float_spec.lua

@@ -8521,6 +8521,34 @@ describe('float window', function()
         ]]}
       end
     end)
+
+    describe('no crash after moving and closing float window #21547', function()
+      local function test_float_move_close(cmd)
+        local float_opts = {relative = 'editor', row = 1, col = 1, width = 10, height = 10}
+        meths.open_win(meths.create_buf(false, false), true, float_opts)
+        if multigrid then
+          screen:expect({float_pos = {[4] = {{id = 1001}, 'NW', 1, 1, 1, true}}})
+        end
+        command(cmd)
+        exec_lua([[
+          vim.api.nvim_win_set_config(0, {relative = 'editor', row = 2, col = 2})
+          vim.api.nvim_win_close(0, {})
+          vim.api.nvim_echo({{''}}, false, {})
+        ]])
+        if multigrid then
+          screen:expect({float_pos = {}})
+        end
+        assert_alive()
+      end
+
+      it('if WinClosed autocommand flushes UI', function()
+        test_float_move_close('autocmd WinClosed * ++once redraw')
+      end)
+
+      it('if closing buffer flushes UI', function()
+        test_float_move_close('autocmd BufWinLeave * ++once redraw')
+      end)
+    end)
   end
 
   describe('with ext_multigrid', function()