@jtanx when I use the ruler tool in the current master (Arch Linux, debug build) I get an assertion failure just when the tool snaps to the second point: fontforge: /home/skef/src/ff3/gdraw/ggdkcdraw.c:52: _GGDKDraw_CheckAutoPaint: Assertion 'false && "No window should be drawing outside of expose calls"' failed.

I've only tried with a couple files (always cubic so far) but it seems to happen every time.