xcb: QDockWidget cannot hide properly a title bar when content widget is a native window

To hide a title bar in QDockWidget I use QWidget with sizeHint() set to QSize(0,0). After switching from title bar to this empty widget when content widget is a native window the title bar exists and it has 1px height. The title bar widget is also a native window (Qt creates it automatically).
With Qt4 it works correctly.

The attachment contains simple test code:

• compile and run with Qt 5.x,
• double click on window to show on full screen,
• press "Shift+L" to hide the title bar,
• move mouse to the top of the screen (screen y=0),
• notice that on status bar it shows mouse events from title widget.

This issue also happens using QGLWidget as dock widget content.