### 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
main_vbox->add_child(title_bar);
	left_l_hsplit = memnew(HSplitContainer);
	main_vbox->add_child(left_l_hsplit);
	left_l_hsplit->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	left_l_vsplit = memnew(VSplitContainer);
	left_l_hsplit->add_child(left_l_vsplit);
	dock_slot[DOCK_SLOT_LEFT_UL] = memnew(TabContainer);
	left_l_vsplit->add_child(dock_slot[DOCK_SLOT_LEFT_UL]);
	dock_slot[DOCK_SLOT_LEFT_BL] = memnew(TabContainer);
	left_l_vsplit->add_child(dock_slot[DOCK_SLOT_LEFT_BL]);
	left_r_hsplit = memnew(HSplitContainer);
	left_l_hsplit->add_child(left_r_hsplit);
	left_r_vsplit = memnew(VSplitContainer);
	left_r_hsplit->add_child(left_r_vsplit);
	dock_slot[DOCK_SLOT_LEFT_UR] = memnew(TabContainer);
	left_r_vsplit->add_child(dock_slot[DOCK_SLOT_LEFT_UR]);
	dock_slot[DOCK_SLOT_LEFT_BR] = memnew(TabContainer);
	left_r_vsplit->add_child(dock_slot[DOCK_SLOT_LEFT_BR]);
	main_hsplit = memnew(HSplitContainer);
	left_r_hsplit->add_child(main_hsplit);
	VBoxContainer *center_vb = memnew(VBoxContainer);
	main_hsplit->add_child(center_vb);
	center_vb->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	center_split = memnew(VSplitContainer);
	center_split->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	center_split->set_collapsed(false);
	center_vb->add_child(center_split);
	right_hsplit = memnew(HSplitContainer);
	main_hsplit->add_child(right_hsplit);
	right_l_vsplit = memnew(VSplitContainer);