@JonB Not a big viewer at all. viewer is a simple QGraphicsView containing a QPixmapItem which is continually updated through another slot taking a signal from the camera thread every time it has a new frame, at 60fps.

Part of the issue seems to be with the debug build, which is obviously a fair bit slower than release. In release it doesn't really seem noticeable, though I imagine it might be on a slow system. Still, I do find the asynchronous behaviour of raise() interesting. I tried looking a bit at the source code, but didn't really get anywhere. Because it's not a problem in release, I guess I'll just leave it be for now. Thanks for the help, though, much appreciated.

I guess if there had been concern of a leak, that was apparently a false positive of a unit test. (I'm not sure about this detail).
This has been resolved, as of today. Here is a summary:

This bug originally appeared in Qt 5.0, was reported in Qt 5.3.0 (QTBUG-39147), and fixed for Qt 5.9.2 (on 7-19-2017). The removal of Qt_WS_* macros (replaced with Q_OS_* macros), and incomplete porting of the widgets/kerna/qtooltip.cpp module, caused the QTipLabel to be instantiated with the client widget as its parent widget. That had been a known problem on Windows which directly caused this dysfunction (Showing a tooltip raises the whole window).

See "Patch Set 6" in this Qt code review page, and it's associated "gitweb" link, and the original bug report:

https://codereview.qt-project.org/#/c/177376/ https://bugreports.qt.io/browse/QTBUG-39147