### 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
right_hsplit->add_child(right_l_vsplit);
	dock_slot[DOCK_SLOT_RIGHT_UL] = memnew(TabContainer);
	right_l_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_UL]);
	dock_slot[DOCK_SLOT_RIGHT_BL] = memnew(TabContainer);
	right_l_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_BL]);
	right_r_vsplit = memnew(VSplitContainer);
	right_hsplit->add_child(right_r_vsplit);
	dock_slot[DOCK_SLOT_RIGHT_UR] = memnew(TabContainer);
	right_r_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_UR]);
	dock_slot[DOCK_SLOT_RIGHT_BR] = memnew(TabContainer);
	right_r_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_BR]);
	
	vsplits.push_back(left_l_vsplit);
	vsplits.push_back(left_r_vsplit);
	vsplits.push_back(right_l_vsplit);
	vsplits.push_back(right_r_vsplit);
	hsplits.push_back(left_l_hsplit);
	hsplits.push_back(left_r_hsplit);
	hsplits.push_back(main_hsplit);
	hsplits.push_back(right_hsplit);
	for (int i = 0; i < vsplits.size(); i++) {
		vsplits[i]->connect("dragged", callable_mp(this, &EditorNode::_dock_split_dragged));
		hsplits[i]->connect("dragged", callable_mp(this, &EditorNode::_dock_split_dragged));
	}
	dock_select_popup = memnew(PopupPanel);
	gui_base->add_child(dock_select_popup);
	VBoxContainer *dock_vb = memnew(VBoxContainer);
	dock_select_popup->add_child(dock_vb);
	HBoxContainer *dock_hb = memnew(HBoxContainer);
	dock_tab_move_left = memnew(Button);