### 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
dock_select->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	dock_vb->add_child(dock_select);
	if (!SceneTree::get_singleton()->get_root()->is_embedding_subwindows() && EDITOR_GET("interface/multi_window/enable")) {
		dock_float = memnew(Button);
		dock_float->set_icon(theme->get_icon("MakeFloating", "EditorIcons"));
		dock_float->set_text(TTR("Make Floating"));
		dock_float->set_focus_mode(Control::FOCUS_NONE);
		dock_float->set_h_size_flags(Control::SIZE_SHRINK_CENTER);
		dock_float->connect("pressed", callable_mp(this, &EditorNode::_dock_make_selected_float));
		dock_vb->add_child(dock_float);
	}
	dock_select_popup->reset_size();
	for (int i = 0; i < DOCK_SLOT_MAX; i++) {
		dock_slot[i]->set_custom_minimum_size(Size2(170, 0) * EDSCALE);
		dock_slot[i]->set_v_size_flags(Control::SIZE_EXPAND_FILL);
		dock_slot[i]->set_popup(dock_select_popup);
		dock_slot[i]->connect("pre_popup_pressed", callable_mp(this, &EditorNode::_dock_pre_popup).bind(i));
		dock_slot[i]->set_drag_to_rearrange_enabled(true);
		dock_slot[i]->set_tabs_rearrange_group(1);
		dock_slot[i]->connect("tab_changed", callable_mp(this, &EditorNode::_dock_tab_changed));
		dock_slot[i]->set_use_hidden_tabs_for_min_size(true);
	}
	editor_layout_save_delay_timer = memnew(Timer);
	add_child(editor_layout_save_delay_timer);
	editor_layout_save_delay_timer->set_wait_time(0.5);
	editor_layout_save_delay_timer->set_one_shot(true);
	editor_layout_save_delay_timer->connect("timeout", callable_mp(this, &EditorNode::_save_editor_layout));
	top_split = memnew(VSplitContainer);
	center_split->add_child(top_split);
	top_split->set_v_size_flags(Control::SIZE_EXPAND_FILL);