bug#72830: Big rectangular selections are slow

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

29 aug. 2024 kl. 22.04 skrev Stefan Monnier <monnier@iro.umontreal.ca>

> We should investigate the case(s) where it doesn't work well and fix
> them (not by changing your `pre-redisplay-function` but by re-running
> that hook), because these cases likely affect other users of this hook
> (such as the normal region highlighting).

What would the problem be for normal (contiguous) region highlighting? There is a single overlay, so there should be no particular cost for large selections. Is there?

The latest patch posted seems to work well enough and all but eliminates the problem (when `select-active-regions` is nil, anyway), so that it outweighs some rare cosmetic issues.

(Those cosmetic issues may occur when a rectangular selection straddles invisible text, because this invalidates the assumption that the other end of the selection is at most (window-height) away. But the selection still operate on the invisible test, so I don't think it's a problem in practice.)

I think the patch is good enough for master. If you have an even better solution, perhaps it could be made as a subsequent improvement?