This looks like a regression between 5.12.0 and 5.12.1. When the user clicks in a widget while the application is non-responsive, a mousePressEvent is delivered after the application becomes responsive again. The corresponding mouseReleaseEvent isn't delivered.

Sample code is attached. During the first 5 seconds while the application is responsive it works normally. Wait until the cursor starts to spin and click. After the second 5 seconds, only the press event is delivered.