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

[Dmitry Gutov <dgutov@yandex.ru>]

On 08/21/2014 06:57 PM, Eli Zaretskii wrote:

>> Hmm. Maybe the fix could be to make the `invisible' property disable
>> `display': as long as former is present, the latter won't work.
>
> That would undoubtedly cause complaints from heavy users of those,
> like Org, magit, etc.

Some change in code might be required, but the resulting code can be 
made backward-compatible, aside from the pathological examples, which 
might improve.

>> After all, that was the intention behind the code I encountered this bug
>> in. And with the current logic, like you say, if `display' is set,
>> `invisible' is redundant.
>
> Yes, but not the other way around.

If the `invisible' starts even one character earlier, it *is* the other 
way around. So one might say that in general case it already disables 
`display'. Try this small modification of the original example:

(let ((pt (point)))
   (insert (propertize "a" 'display "bbb"))
   (let ((o (make-overlay (1- pt) (point))))
     (overlay-put o 'invisible t)
     (overlay-put o 'after-string "foo\nbar")))

The result looks like:

foo
bar

>> Thanks. That looks very much like a bug as well, though maybe again, too
>> expensive to fix. FWIW, for that issue, if myov2 has higher priority
>> than myov1 (if only by virtue of being inside and shorter), I'd display
>> just "STRING2" ("STRING1" would not be visible at all). But that's just
>> going by logic; maybe there's a use case that would break.
>
> I'm quite sure there's some use case somewhere that will break.

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