### 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
if (!get_viewport()) {
		return;
	}
	Size2 size = get_size() * get_viewport()->get_canvas_transform().get_scale();
	popup->set_size(Size2(size.width, 0));
	Point2 gp = get_screen_position();
	gp.y += size.y;
	if (is_layout_rtl()) {
		gp.x += size.width - popup->get_size().width;
	}
	popup->set_position(gp);
	popup->popup();
}
void ScreenSelect::pressed() {
	if (popup->is_visible()) {
		popup->hide();
		return;
	}
	_build_advanced_menu();
	_show_popup();
}
ScreenSelect::ScreenSelect() {
	set_tooltip_text(TTR("Make this panel floating.\nRight click to open the screen selector."));
	set_button_mask(MouseButtonMask::RIGHT);