Comment 6 for bug 1300664

Well I've made some progress... I need to dump my brain because it's getting full and I have other stuff to do for a while.

First, this problem existed in the 1.11.0 release, although it may have been less noticeable. Second, as long as you keep the mouse moving quickly it doesn't happen. It's only when you hold a drag still.

The problem is that the GUI thread is waiting. (And the VSync thread is waiting on the GUI thread via semaphores, but that's not relevant.) There's plenty of CPU available too, so something is blocking. I ran the Very Sleepy profiler and you can see in the attached screenshot that "DoDragDrop" in ole32 is the culprit. This probably explains why the problem doesn't affect Linux (ole32 is a standard windows module).

This really looks like a flaw in Qt - I'm pretty sure they did not want to make a blocking call here. But perhaps they have no choice on Windows?

Does anyone have a Qt5 Windows version they can test? It would be interesting to see if the problem exists with Qt5.