### 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
EditorProgress save("save", TTR("Saving Scene"), 4);
	if (editor_data.get_edited_scene_root() != nullptr) {
		save.step(TTR("Analyzing"), 0);
		int c2d = 0;
		int c3d = 0;
		_find_node_types(editor_data.get_edited_scene_root(), c2d, c3d);
		save.step(TTR("Creating Thumbnail"), 1);
		
		Ref<Image> img;
		
		
		
		if (c3d == 0 && c2d == 0) {
			img.instantiate();
			img->initialize_data(1, 1, false, Image::FORMAT_RGB8);
		} else if (c3d < c2d) {
			Ref<ViewportTexture> viewport_texture = scene_root->get_texture();
			if (viewport_texture->get_width() > 0 && viewport_texture->get_height() > 0) {
				img = viewport_texture->get_image();