bug#40821: Margin strings are displayed in reverse order of overlay priority (low-priority specs hide high-priority ones)

[Clément Pit-Claudel]

On 25/04/2020 10.04, Eli Zaretskii wrote:
>> I only want/need to display one margin indicator — the highest priority one 
>> :)
> 
> Then why not have your code do that?

It's quite complicated: every time I add a new overlay (they arrive 
asynchronously, not all at once), I need to check for others on the same line 
and adjust the symbols.  Every time the text is changed, I need to rescan the 
current line and adjust the overlay properties.

> It is your Lisp program that puts these overlays, isn't it?  Why
> cannot it put only one overlay, the one you want to be displayed in
> this case?

Because both are needed: for example, when the point is on ewew, the help-echo 
will be the concatenation of both.  Similarly, when the user browses the error 
list in a separate buffer, highlighting one error will light up the 
corresponding portion of the buffer.  Additionally, users can filter errors, 
hiding certain overlays.

>> I thought this would show a fringe icon in compilation-error face, because 
>> of the higher priority of the corresponding overlay.  It doesn't.
>> Should I report this as a separate issue?
> 
> Why is that an issue?  You in effect invoke undefined behavior, and
> the result is not outlandish, IMO.

It is: the buffer shows the face of one overlay and the icon of another one.

>> Widening the margin isn't a working solution for this case, but displaying 
>> the margin specs in order of increasing priority would work.  However, 
>> wouldn't that require a second pass as well?  That is, if margin specs were 
>> sorted by priority before being inserted, it would work (it wouldn't matter 
>> that there are multiple instead of one, since the most important one would 
>> be first).
> 
> Now I'm confused: the overlays are already being sorted by the display
> engine, and displayed in the order of increasing priority, as I
> explained in my original message.

The margin specs are displayed in order of decreasing priority, as far as I can 
tell: first the one coming from the overlay with the lowest priority, then the 
next one, etc.