Since upgrading to Big Sur, I noticed that the volume control doesn't behave as it always has on 2.7.4 and 2.7.6
Moving the mouse up causes the volume slider to go down and vice versa.
There is a very opaque mention of some breakage in vertical QSlider on Big Sur in https://bugreports.qt.io/browse/QTBUG-86513 which should have been already fixed upstream. I could not find any details about code changes related to the fix. Hopefully, it will find its way into the next Qt 5.15.x maintenance release.
Thanks for the info!
Sadly this is still an issue with the latest release 2.7.8 today. Volume is upside down/messed up.
It doesn't look like Qt has bothered to fix the issue in Qt5.