I made some further tests and I think the problem I had yesterday is actually caused by two things:
(i) the WiFi connection was quite unstable yesterday. Thus with better WiFi I could not reproduce the “line-drawing-to-the-border” problem today.
(ii) the notification bar is still touchable and you can activate it accidentally.
While (i) is totally my fault, (ii) should probably be fixed. Maybe you can do it in the same way as it is done with the system’s video player, i.e., if the power button is pressed the touch interface is locked.