The only way I can get the cancel box to show while scanning is for me to move the windows toolbar from the bottom to the right hand side, then I hold on to the GUI top bar and pull it up so it then gives me a little more space on the GUI to get the cancel box to show, I then tab to this box till I see it is highlighted, I then let go of the upper bar then I hit enter, this then cancels the scanning.
Shouldn't have to do this of course so I hope this bug is fixed on the next stable release.