V10.13.16 introduced the ability to click-hold on the framestep skill panel buttons. V10.13.16-B appears to have broken this. Clicking to advance / rewind one frame at a time, as well as the right-click for 1 second / middle-click for 5 seconds, are still working.
This was quite a nasty one to track down the cause of (though easily fixed once the cause was known), but it's fixed for the next update. It actually appears this was broken before V10.13.16's release even.