[X11][Reg 5.5.1 -> 5.6.0-beta] Application output flooded with "Attempt to set a screen on a child window" when using native windows

When having a native window in a dock widget every resize, move, undock or similar action triggers a flood of

0x1373da0 ( QScreen(0x13308e0, name="default") ): Attempt to set a screen on a child window

messages in the application output on X11 platforms (tested on Ubuntu 14.04 and RHEL6). This doesn't happen with Qt 5.5.1.

The attached example demonstrates the problem.