I have underestimated the performance implications of evaluating everything as lazy as possible in QtQuick/QML. The col/row-numbers get their color from calculating if there are too many walls in them or not. This routine got executed on each mouse-drag, maybe even on every hover. I added two arrays that hold the count-states, and changed the update-invocations to only happen on adding/removing of a wall.

It may sound like premature optimization, but all levels bigger than 6x6 were really tedious to play on iOS, simulator and iPhone.