### 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
distraction_free->set_toggle_mode(true);
	tabbar_container->add_child(distraction_free);
	scene_root_parent = memnew(PanelContainer);
	scene_root_parent->set_custom_minimum_size(Size2(0, 80) * EDSCALE);
	scene_root_parent->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("Content"), SNAME("EditorStyles")));
	scene_root_parent->set_draw_behind_parent(true);
	srt->add_child(scene_root_parent);
	scene_root_parent->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	scene_root = memnew(SubViewport);
	scene_root->set_embedding_subwindows(true);
	scene_root->set_disable_3d(true);
	scene_root->set_disable_input(true);
	scene_root->set_as_audio_listener_2d(true);
	main_screen_vbox = memnew(VBoxContainer);
	main_screen_vbox->set_name("MainScreen");
	main_screen_vbox->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	main_screen_vbox->add_theme_constant_override("separation", 0);
	scene_root_parent->add_child(main_screen_vbox);
	bool global_menu = !bool(EDITOR_GET("interface/editor/use_embedded_menu")) && DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_GLOBAL_MENU);
	bool can_expand = bool(EDITOR_GET("interface/editor/expand_to_title")) && DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_EXTEND_TO_TITLE);
	if (can_expand) {
		
		left_menu_spacer = memnew(Control);
		left_menu_spacer->set_mouse_filter(Control::MOUSE_FILTER_PASS);
		title_bar->add_child(left_menu_spacer);
	}
	main_menu = memnew(MenuBar);
	title_bar->add_child(main_menu);
	main_menu->add_theme_style_override("hover", gui_base->get_theme_stylebox(SNAME("MenuHover"), SNAME("EditorStyles")));
	main_menu->set_flat(true);