### 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 (p_idx == 0) { 
		current_menu_option = FILE_EXPORT_MESH_LIBRARY;
		if (!editor_data.get_edited_scene_root()) {
			show_accept(TTR("This operation can't be done without a scene."), TTR("OK"));
			return;
		}
		List<String> extensions;
		Ref<MeshLibrary> ml(memnew(MeshLibrary));
		ResourceSaver::get_recognized_extensions(ml, &extensions);
		file_export_lib->clear_filters();
		for (const String &E : extensions) {
			file_export_lib->add_filter("*." + E);
		}
		file_export_lib->popup_file_dialog();
		file_export_lib->set_title(TTR("Export Mesh Library"));
	} else { 
		if (export_as_menu->get_item_submenu(p_idx).is_empty()) { 
			Callable callback = export_as_menu->get_item_metadata(p_idx);
			Callable::CallError ce;
			Variant result;
			callback.callp(nullptr, 0, result, ce);
			if (ce.error != Callable::CallError::CALL_OK) {
				String err = Variant::get_callable_error_text(callback, nullptr, 0, ce);
				ERR_PRINT("Error calling function from export_as menu: " + err);
			}
		}
	}
}
int EditorNode::_next_unsaved_scene(bool p_valid_filename, int p_start) {