bug#57150: 29.0.50; [PATCH] Add test coverage for overlay modification hooks

[Ihor Radchenko <yantar92@gmail.com>]

Richard Stallman <rms@gnu.org> writes:

> The first question is, should these use text properties instead of
> overlays?

It may be possible, but there are non-trivial implications when one
attempts to replace overlays with text properties. Some
implications I can remember quickly:

(1) Overlays do not overwrite original properties and thus removing
    overlays automatically recovers the original properties. Not so
    easily when using text properties.
(2) Overlays are not copied when the text is killed. Text properties
    are. Overlays are also not carried between indirect buffers.
(3) Inserting text inside overlays automatically inherits their
    properties. To do the same with text properties, one must use
    special functions like `insert-and-inherit', which is not common in
    packages.

To conclude, overlays and text-properties are similar in many aspects,
but also handled differently in other aspects. These differences make it
very annoying to change from overlays to text property approaches.

> Are they using many overlays to highlight many potential matches in a
> long buffer?  If so, would it make sense to show, at any time, only those
> that are near point?  When you move to a far-away part of the buffer,
> it could add highlighting to the matches near there.  But maybe it
> can avoid ever trying to highlight more than a few parts of the buffer.

I guess it can be a valid optimization in some cases. But it is not
always possible. For example, consider folded headlines in Org mode.
They have some initial folding state set when opening the file, but the
state can be changed any time by the user. It is technically challenging
to put overlays on the fly in such scenario.

-- 
Ihor Radchenko,
Org mode contributor,
Learn more about Org mode at https://orgmode.org/.
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