[REG 5.3.2->5.4] XCB: QDockWidget invisible after setFloating(true and changing title bar widget

I was testing the latest Qt 5.4 RC snapshot on Linux and experienced a strange bug with my dockable widgets.

The main application consists of several dock widgets. As soon as I'm trying to float one of the widgets, they are undocked from the main widget and are invisible. If I dock them back, everything is fine.

See attached screenshots from GammaRay, while trying to find out what happened.

The screenshot below shows the default window after the application had started. The widget with the red border is a QDockWidget and will be floated in the next screenshot.

Below you can see what happens when the QDockWidget was floated. The position was moved out of the screen completly. I was able to move it back with Gammaray and saw the position with the red border. But its not rendered. Any attempt to change position, size or visibility in code failed as well.

The last screenshot is what I get when using Qt 5.3.2. The undocked widget is rendered properly.

Was there any change in 5.4 which might explain this? Unfortunately I could not yet produce a minimal example. All QDockWidget examples from the repository seem to work fine.

I don't yet if this works on other platforms but will test it.