[p_l]

Screen tearing is not really an X11 issue. It's an XFree86/X.Org issue in that they are to this day in many ways the lowest common denominator implementations, and at some point Intel removed VSync circuitry from their IGPs completely - so the only way to prevent screen tearing was to have global compositor doing page flips. So, a lot of that "never noticed screen tearing" is due to people running different GPUs and/or enabling some driver features mitigating vendor weirdness.

Similarly, nothing says you need to request a redraw when moving a window instead of redrawing it from backing store - but at some point backing store became useless in X.Org even if you explicitly try to program its use.