X: 25 years and still lacking vsync'ed double buffering

If all drawing to the screen happens to a back buffer and that buffer is only flipped into view during retrace then tearing cannot happen.

You are entirely correct that the entire rendering chain has to be synchronized and driven by the retrace, if you want the best possible result, but it would help a great deal if X could simply stay away from the on-screen buffer until there was a retrace.

Tearing is by far the most nasty artifact that comes from sloppy graphics work, but an application that jumps between full framerate and half framerate (say 70 and 35 Hz) looks almost as bad, so setting up a proper synchronized rendering chain would certainly be worth doing.

I don't think any of this would have to eat truck loads of memory, though, you'd certainly need to have a number (3 or more) frames buffered, but that's nothing compared to the memory on common graphics cards these days.

The buffering might not have to happen for the entire screen, you could easily have a single application (a video player, perhaps) that's the only thing that really needs to sync to vtrace and run at a fixed framerate to look good, that single app. could have a deep set of buffers, everything else could get by with the global tripple bufer.