If, like me, you're trying to build something that requires you to do something every X blocks, but despise having to hold F3 to see your coordinates...
Click and hold F3
Press ALT and release
Release F3
Press ALT and release
The debug information should stay onscreen until you hit F3 again. For now, anyway.
Totally helped expanding my skyway ~1000 blocks westward, needing to know every 10th block