### Godot version

v4.2.dev.custom_build [0c2144da9]

### System information

Debian Linux X11 Xfce

### Issue description

Prerequisite: Be in Single Window Mode in the Editor
The Filesystem Dock is unusable when it is floating, which is not expected.
It appears garbled in the top-left corner:
![image](https://github.com/godotengine/godot/assets/6299227/109f7922-97de-44c3-814a-ca7f970e0602)

The following errors are shown:
```
  editor/window_wrapper.cpp:220 - Condition "!is_window_available()" is true.
  editor/window_wrapper.cpp:202 - Condition "!get_window_enabled()" is true. Returning: Rect2i()
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
```

bisecting lead to #62378
possibly related to #44439

### Steps to reproduce

1. Start a project in the Editor
2. Switch to Single Window Mode in the Editor and reload (if necessary)
3. Make the Filesystem Dock floating

### Minimal reproduction project

Editor issue
} else {
					show_save_accept(vformat(TTR("%s no longer exists! Please specify a new save location."), scene->get_scene_file_path().get_base_dir()), TTR("OK"));
				}
				break;
			}
			[[fallthrough]];
		}
		case FILE_SAVE_AS_SCENE: {
			int scene_idx = (p_option == FILE_SAVE_SCENE || p_option == FILE_SAVE_AS_SCENE) ? -1 : tab_closing_idx;
			Node *scene = editor_data.get_edited_scene_root(scene_idx);
			if (!scene) {
				if (p_option == FILE_SAVE_SCENE) {
					
					
					ScriptEditor::get_singleton()->save_current_script();
				}
				const int saved = _save_external_resources();
				if (saved > 0) {
					show_accept(
							vformat(TTR("The current scene has no root node, but %d modified external resource(s) were saved anyway."), saved),
							TTR("OK"));
				} else if (p_option == FILE_SAVE_AS_SCENE) {
					
					show_accept(
							TTR("A root node is required to save the scene. You can add a root node using the Scene tree dock."),
							TTR("OK"));
				}
				break;
			}
			file->set_file_mode(EditorFileDialog::FILE_MODE_SAVE_FILE);