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bug#18285: 24.3.92; A combination of `display' on text and `invisible' a

From: Dmitry Gutov
Subject: bug#18285: 24.3.92; A combination of `display' on text and `invisible' and `before/after-string' leads to the before/after string being displayed twice
Date: Fri, 22 Aug 2014 04:58:55 +0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0

On 08/21/2014 08:06 PM, Eli Zaretskii wrote:

If the `invisible' starts even one character earlier, it *is* the other
way around.

Yes, because then there's no doubt about the order of evaluating the
various properties and acting upon them.  By contrast, when they all
begin at the same buffer position, the order is
implementation-defined.  The code is written to examine display
properties before the invisible properties.

Okay, but I'll take this to mean that hitting the changed behavior in existing code would be pretty rare.

Anyway, how about the other way around? I'll like this less, but why not make `invisible' inactive when `display' is set?

This works is a more consistent fashion, and the text that would be invisible ("a") is replaced by `display' anyway:

(let ((pt (point)))
  (insert (propertize "a" 'display "bbb"))
  (let ((o (make-overlay pt (point))))
    (overlay-put o 'after-string "foo\nbar")))

looks like


Maybe. But at least it's consistent with the overlay priority rules.

The priority is _per_buffer_position_.  We tend to forget that text
properties and overlays in Emacs are _character_ properties, but the
display engine is designed and implemented to support that, and in
some obscure cases, like this one, it is impossible to understand its
logic, unless one remembers this simple fact.

I'm just quoting Stefan from that discussion:

Same problem: for two overlays of equal `priority', the shorter one has
higher priority, so your original example is already one of those
cases, AFAIC.

Like in this example:

(let ((pt (point)))
  (insert "aaa")
(let ((o (make-overlay pt (point))) (v (make-overlay (1+ pt) (1- (point)))))
    (overlay-put o 'face 'bold)
    (overlay-put v 'face 'default)))

the middle character has normal weight, even though it's also covered by an outer overlay that sets `face' to `bold'.

So, if I had to pick which single string to show (STRING1 or STRING2), the latter seems to be the more consistent choice.

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