In 5.11.1 Puzzle example works fine with touch, but in Qt 5.12.0-alpha1 it is stuck in drag mode until mouse is moved on top of the window.
Seems that there was an attempt to fix this exact issue for Linux, not sure if that is related:
|For Gerrit Dashboard: QTBUG-70887|
|241865,3||Windows QPA: Make Windows Pointer Input Messages opt-in||5.12||qt/qtbase||Status: ABANDONED||-1||0|
|242911,5||Windows QPA: Fix Drag&Drop from touchscreen/pen||5.12||qt/qtbase||Status: MERGED||-2||0|