[Reg 5.5->5.6] QDockWidget with custom title widget cannot be resized when floating

After setting a custom title widget (setTitleBarWidget) on a QDockWidget, it is not resizable while it is floating/undocked.

Reproducible with the "blue" dock widget in the standard main window example.

This seems to be a regression in Qt 5.6.3 and later, since supposedly this worked in Qt 5.5 (QTBUG-47977) and I was able to reproduce with 5.6.3, 5.9.x and 5.12.0+.1.

This affects the debugger dock widgets in Qt Creator.