I'm maintaining Qt for a linux distribution and just upgraded from Qt5.9.x to Qt5.15.0 (and now Qt 5.15.1).
For one particular platform - IMX6 + qt5.15.1 + 2d software renderer + linuxfb + touch (evdev (without mtdev)) I'm experiencing erratic behaviour with the touch handling.
Once the faulty behaviour can be triggered it can be reproduced all the time and doesn't seem to change. Same with the working one.
The issue is best observed in a SwipeDelegate, where I'm not or only 1 out of 10 times able to scroll /vertically/. Touch drags are mostly considered (that's only an assumption from what I observe, though) /horizontally/.
The behaviour changes (between as stated (erratic) and expected) by modifications throughout the whole system.
Variables are glibc vs. muslibc, debugging enabled/disabled, even just swipe-delegate's filename ("/opt/XXX" vs "/opt/XXXXX") - just to state some examples demonstrating its randomness.
To focus on a particular change reproducibly toggling the behaviour without doing persistent changes:
QT_LOGGING_RULES=qt.qpa.input=false -> not working
QT_LOGGING_RULES=qt.qpa.input=true > working-
So it all looks like a race condition somewhere to me, yet - even after having read the (seemingly) responsible code several times - I couldn't narrow it down.
I attached two videos showing the working/expected and the erratic version.
The erratic version also shows, that I can scroll just fine vertically, if i'm dragging from a space not (anymore) occupied by list elements. Dragging from the list items directly, though, is problematic.