Qt5 GUI on Ubuntu is broken in master again. Moving the mouse pointer leads to a situation where the coordinate labels start lagging behind the movement. Git bisect pointed to commit 604299f.

I have not looked into the details yet, but it looks like there is a calculation happening that might take a little bit more CPU time after the patch. This alone shouldn't be an issue, but apparently this code path is again executed synchronously by the GUI event handler at high frequency, which should not be happening. The patch isn't responsible for this, the previous calculation probably just didn't cause the event loop to clog up.