MDI Child Window not resizes correctly

When creating new widgets (children) into an existing MDI SubWindow the Subwindow does not adjust it's size correctly.
The MDI Widget is updated when user moves or resizes the widget.

The attached example shows the problem by creating an empty MDI child. After one second a button is added
but the window does not resize correctly. (Try to click to the title bar with the mouse and the window will resize properly.)