Fix Screen::_cuY position inside screen

Keep _cuY position inside max screen lines in AppScreen mode.

src/Screen.cpp

@@ -419,6 +419,7 @@ void Screen::setCursorLine(int newLine)
 {
     if (_currentModes[MODE_AppScreen] == 1) {
         _savedState.cursorLine = newLine;
+        _cuY = qBound(0, _cuY, _lines - 1);
     } else {
         _cuY = newLine;
     }