### 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
editor_layout_save_delay_timer->start();
}
void EditorNode::_dock_split_dragged(int ofs) {
	editor_layout_save_delay_timer->start();
}
void EditorNode::_load_editor_layout() {
	Ref<ConfigFile> config;
	config.instantiate();
	Error err = config->load(EditorPaths::get_singleton()->get_project_settings_dir().path_join("editor_layout.cfg"));
	if (err != OK) { 
		
		TreeItem *root = FileSystemDock::get_singleton()->get_tree_control()->get_item_with_metadata("res:", 0);
		if (root) {
			root->set_collapsed(false);
		}
		if (overridden_default_layout >= 0) {
			_layout_menu_option(overridden_default_layout);
		}
		return;
	}
	_load_docks_from_config(config, "docks");
	_load_open_scenes_from_config(config);
	_load_central_editor_layout_from_config(config);
	editor_data.set_plugin_window_layout(config);
}
void EditorNode::_update_dock_slots_visibility(bool p_keep_selected_tabs) {