How do I give an overlay a priority lower to that of hl-line-mode?

How do I give an overlay a priority lower to that of hl-line-mode?

[Clément Pit--Claudel]

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Hi emacs-dev,

A mode that I contribute to uses overlays to highlight certain lines. The added overlays typically cover (point-at-bol) .. (1+ (point-at-eol)), which unfortunately does not play nicely with hl-line-mode: my overlays hide the one added by hl-line-mode.

Demo:

(with-current-buffer (get-buffer-create "*hl*")
  (erase-buffer)
  (fundamental-mode)
  (hl-line-mode)
  (dotimes (_ 5) (insert "AAAAAAAAAA\n"))
  (goto-char 25)
  (let ((ov (make-overlay (point-at-bol) (1+ (point-at-eol)))))
    (overlay-put ov 'face '(:background "red")))
  (pop-to-buffer (current-buffer)))

Of course, the temptation is great to add a little (overlay-put ov 'priority -100), but it's explicitly a bad idea:

> This property’s value determines the priority of the overlay. If you
> want to specify a priority value, use either nil (or zero), or a
> positive integer. Any other value has undefined behavior.

What alternatives do I have? (In fact, do I have any?)

Thanks!


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Re: How do I give an overlay a priority lower to that of hl-line-mode?

[Stefan Monnier]

> What alternatives do I have? (In fact, do I have any?)

Alternatives:
- use text-properties instead of overlays.
- undocumented and tentatively added in recentish Emacsen you can use
  a priority of the form (nil . -100).  Which means: normal priority nil
  in general, but priority -100 when the normal priority rules (which
  look at the relative size of overlays) don't have a preference
  use -100.
- use a negative priority.


        Stefan

Re: How do I give an overlay a priority lower to that of hl-line-mode?

[Clément Pit--Claudel]

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On 2016-05-31 16:28, Stefan Monnier wrote:
>> What alternatives do I have? (In fact, do I have any?)
> 
> Alternatives:

Thanks Stefan!

> - use text-properties instead of overlays.

Could work, indeed. Though adding and removing faces in text properties is trickier.

> - undocumented and tentatively added in recentish Emacsen you can use
>   a priority of the form (nil . -100).  Which means: normal priority nil
>   in general, but priority -100 when the normal priority rules (which
>   look at the relative size of overlays) don't have a preference
>   use -100.

Oh, neat :) Do you know in which version this was introduced?

> - use a negative priority.

Does this just behave like positive priorities, in practice?

Clément.


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Re: How do I give an overlay a priority lower to that of hl-line-mode?

[Stefan Monnier]

>> - undocumented and tentatively added in recentish Emacsen you can use
>> a priority of the form (nil . -100).  Which means: normal priority nil
>> in general, but priority -100 when the normal priority rules (which
>> look at the relative size of overlays) don't have a preference
>> use -100.
> Oh, neat :) Do you know in which version this was introduced?

IIRC it was added for the overlay-based region-highlight, so it should
be in 24.4.

>> - use a negative priority.
> Does this just behave like positive priorities, in practice?

Yes.  But these priorities are always tricky to use (no matter what
priority you choose, they'll always end up wrong at some point), in
my experience.  So positive or negative, they tend to suck.


        Stefan