### 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 (!is_window_available()) {
		return;
	}
	window->set_title(p_title);
}
void WindowWrapper::set_margins_enabled(bool p_enabled) {
	if (!is_window_available()) {
		return;
	}
	if (!p_enabled && margins) {
		margins->queue_free();
		margins = nullptr;
	} else if (p_enabled && !margins) {
		Size2 borders = Size2(4, 4) * EDSCALE;
		margins = memnew(MarginContainer);
		margins->add_theme_constant_override("margin_right", borders.width);
		margins->add_theme_constant_override("margin_top", borders.height);
		margins->add_theme_constant_override("margin_left", borders.width);
		margins->add_theme_constant_override("margin_bottom", borders.height);
		window->add_child(margins);
		margins->set_anchors_and_offsets_preset(PRESET_FULL_RECT);
	}
}
WindowWrapper::WindowWrapper() {