bug#72830: Big rectangular selections are slow

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>>> - `select-active-regions`: as mentioned, it slows down rectangle selection
>>> massively and is alien to non-X11 platforms so I'd suggest it be set to nil
>>> by default on macOS and Windows at least.
>> I think we should first try and make it not-slow, by making it lazy.
> No objections there but we would need to rework some plumbing. Emacs's behaviour is odd.
> When a selection is made, that text is extracted and squirrelled away just
> in case, so the user can actually select some text, deselect and move
> around, and even make some changes the the buffer, and then paste PRIMARY
> from Emacs or any other X11 client and still get the originally
> selected text.

IIRC from the last time I looked at that code, I got the impression that
the design was made [pun ahead!] primarily for the CLIPBOARD selection
and *should* work something like this:
- when we make a new selection, we tell X11 that we own the CLIPBOARD.
  This should be an O(1) operation.
- when the selection changes because we move point or mark, we don't
  need to do anything.
- we get the content of that selection (an O(N) operation) only if/when
  X11 asks for it.
- in order to still be able to send the CLIPBOARD's content after the
  selection has disappeared, we pay the O(N) cost when the region is
  deactivated and "squirrelled away" (like you say) that content.

So the big rectangular selections should slow down only steps 3 and
4 but not step 1 or 2.  And as you point out, maybe step 4 could/should
be skipped for PRIMARY, tho I'm not sure in which cases it would
be beneficial (beside those cases where the region is so large that the
O(N) cost is a problem).


        Stefan


PS: Of course, if needed, maybe the step 4 could be made faster by
squirreling away not the exact rectangular region, but something that
can be computed more quickly and from which we can later still extract
the exact rectangular region if/when needed.