[Regression 6.5.1->6.5.6]QDocketWidget with custom title widget cannot be resized

A minimal reproducer attached.

I don't have all the Qt6.5.x versions, but I am sure there is something breaks the behavior between 6.5.1 and 6.5.6 (6.5.7 is affected too). On the other hand, 5.15, 6.7 and 6.8 work well. But anyway, build the reproducer using 6.5.1 and you can resize the QDockWidget by dragging its window edge. But you can't do that with 6.5.6 or 6.5.7. The arrow mark does not even show up when you hover cursor over window edge.