Mouse stuck in 'dragging' mode
I had the same issue in multiple applications including Okular, Eagle etc. I accidentally discovered that it might be related to the touchscreen. While in the 'dragging mode' in Okular, I could get out by simply tapping on the touchscreen once. I then experimented by explicitly touching the screen (and not taking my finger off) and immediately, the pointer started dragging. I suspect one possible cause for the behavior is spurious touch detects without the corresponding release being detected.