document viewer window close button and kill (x) button do not do the same thing

[butlerpd]

The kill button, the X at the top right corner of the window calls the QtWidget closeEvent. The Close button calls the local onClose method then calls after checking for open threads self.close.

The two methods of closing the window should behave identically

[jamescrake-merani]

Is this a major issue? I'm not sure if it should go into 6.1.0 given the stage of release we're at.