flutter: keep chat window within screen bounds to prevent hidden chat window (fixes rustdesk#13397) (#13406)

flutter/lib/common/widgets/overlay.dart

@@ -50,6 +50,7 @@ class DraggableChatWindow extends StatelessWidget {
           )
         : Draggable(
             checkKeyboard: true,
+            checkScreenSize: true,
             position: draggablePositions.chatWindow,
             width: width,
             height: height,
@@ -395,7 +396,10 @@ class _DraggableState extends State<Draggable> {
     _chatModel?.setChatWindowPosition(position);
   }
 
-  checkScreenSize() {}
+  checkScreenSize() {
+    // Ensure the draggable always stays within current screen bounds
+    widget.position.tryAdjust(widget.width, widget.height, 1);
+  }
 
   checkKeyboard() {
     final bottomHeight = MediaQuery.of(context).viewInsets.bottom;
@@ -517,6 +521,12 @@ class IOSDraggableState extends State<IOSDraggable> {
     _lastBottomHeight = bottomHeight;
   }
 
+  @override
+  void initState() {
+    super.initState();
+    position.tryAdjust(_width, _height, 1);
+  }
+
   @override
   Widget build(BuildContext context) {
     checkKeyboard();