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bug#459: Zero-length overlays, overlay keymaps, and `overlays-at'

From: Toby Cubitt
Subject: bug#459: Zero-length overlays, overlay keymaps, and `overlays-at'
Date: Tue, 20 Jul 2021 14:30:04 +0000

On Tue, Jul 20, 2021 at 03:46:38PM +0200, Lars Ingebrigtsen wrote:
> > This first bug in the report still seems to be there in current stable
> > Emacs. Is the intention to dismiss that first bug as "won't fix", too?
> > Just flagging, in case that part got missed.
> I didn't check that bit, since you said:
> ----
> Looking through NEWS.22, I came upon the following:
> *** The `keymap' property now also works at the ends of overlays and
> text properties, according to their stickiness.  This also means that it
> works with empty overlays.  The same hold for the `local-map' property.
> So the first issue I reported is already fixed! (In exactly the way I
> suggested, which makes me suspect that future-bug might now have been
> fixed too... :) However, this behaviour isn't documented in the Emacs
> Lisp manual (unless I've overlooked something yet again), which *is* a
> bug.
> ----

No, I'd also forgotten that I wrote that :) I did test before emailing, but 
didn't test with different stickiness. Making it stickiness-dependent makes 

> Oh, I missed the bit about the missing documentation?  Is that what
> you're referring to?

I suppose if it was documented, I might have remembered that it was already 
fixed :) Maybe it is somewhere, but I missed it? The Emacs Lisp manual 
currently (Emacs stable) has this to say on the overlay keymap property:

"This keymap is used when the character after point is within the overlay"

Which is technically incorrect since that NEWS.22 item, as it doesn't cover 
zero-length, null front-advance, non-null rear-advance overlays. A concise, 
technically correct wording that I think covers all cases would be:

"This keymap is used when a character inserted after point would be included 
within the overlay"

But maybe it would be better to be less concise, and spell out the zero-length 
overlay special case separately.

Dr T. S. Cubitt
Reader (Associate Professor) in Quantum Information
Royal Society University Research Fellow
Department of Computer Science
University College London

email: tsc25@cantab.net
web:   www.dr-qubit.org

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