Fix changing `gui_embed_subwindows` while a child window is displayed

Changing `gui_embed_subwindows` while a child window is displayed can lead
to crashes or other unintended behavior.
This PR adds checks to prevent the change while child windows are
displayed.

scene/main/viewport.cpp

@@ -3848,7 +3848,66 @@ Viewport *Viewport::get_parent_viewport() const {
 
 void Viewport::set_embedding_subwindows(bool p_embed) {
 	ERR_THREAD_GUARD;
-	gui.embed_subwindows_hint = p_embed;
+	if (gui.embed_subwindows_hint == p_embed) {
+		return;
+	}
+
+	bool allow_change = true;
+
+	if (!is_inside_tree()) {
+		// Change can happen since no child window is displayed.
+	} else if (gui.embed_subwindows_hint) {
+		if (!gui.sub_windows.is_empty()) {
+			// Prevent change when this viewport has embedded windows.
+			allow_change = false;
+		}
+	} else {
+		Viewport *vp = this;
+		while (true) {
+			if (!vp->get_parent()) {
+				// Root window reached.
+				break;
+			}
+			vp = vp->get_parent()->get_viewport();
+			if (vp->is_embedding_subwindows()) {
+				for (int i = 0; i < vp->gui.sub_windows.size(); i++) {
+					if (is_ancestor_of(vp->gui.sub_windows[i].window)) {
+						// Prevent change when this viewport has child windows that are displayed in an ancestor viewport.
+						allow_change = false;
+						break;
+					}
+				}
+			}
+		}
+
+		if (allow_change) {
+			Vector<int> wl = DisplayServer::get_singleton()->get_window_list();
+			for (int index = 0; index < wl.size(); index++) {
+				DisplayServer::WindowID wid = wl[index];
+				if (wid == DisplayServer::INVALID_WINDOW_ID) {
+					continue;
+				}
+
+				ObjectID woid = DisplayServer::get_singleton()->window_get_attached_instance_id(wid);
+				if (woid.is_null()) {
+					continue;
+				}
+
+				Window *w = Object::cast_to<Window>(ObjectDB::get_instance(woid));
+				if (w && is_ancestor_of(w)) {
+					// Prevent change when this viewport has child windows that are displayed as native windows.
+					allow_change = false;
+					break;
+				}
+			}
+		}
+	}
+
+	if (allow_change) {
+		gui.embed_subwindows_hint = p_embed;
+	} else {
+		WARN_PRINT("Can't change \"gui_embed_subwindows\" while a child window is displayed. Consider hiding all child windows before changing this value.");
+	}
 }
 
 bool Viewport::is_embedding_subwindows() const {