### 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
set_flat(true);
	set_toggle_mode(true);
	set_focus_mode(FOCUS_NONE);
	set_action_mode(ACTION_MODE_BUTTON_PRESS);
	
	const Size2 borders = Size2(4, 4) * EDSCALE;
	popup = memnew(Popup);
	popup->connect("popup_hide", callable_mp(static_cast<BaseButton *>(this), &ScreenSelect::set_pressed).bind(false));
	add_child(popup);
	popup_background = memnew(Panel);
	popup_background->set_anchors_and_offsets_preset(PRESET_FULL_RECT);
	popup->add_child(popup_background);
	MarginContainer *popup_root = memnew(MarginContainer);
	popup_root->add_theme_constant_override("margin_right", borders.width);
	popup_root->add_theme_constant_override("margin_top", borders.height);
	popup_root->add_theme_constant_override("margin_left", borders.width);
	popup_root->add_theme_constant_override("margin_bottom", borders.height);
	popup->add_child(popup_root);
	VBoxContainer *vb = memnew(VBoxContainer);
	vb->set_alignment(BoxContainer::ALIGNMENT_CENTER);
	popup_root->add_child(vb);
	Label *description = memnew(Label(TTR("Select Screen")));
	description->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
	vb->add_child(description);
	screen_list = memnew(HBoxContainer);
	screen_list->set_alignment(BoxContainer::ALIGNMENT_CENTER);
	vb->add_child(screen_list);
	popup_root->set_anchors_and_offsets_preset(PRESET_FULL_RECT);
}