bug#4911: mouse-face property should merge face attributes, not replace

[Clément Pit-Claudel]

On 08/05/2020 11.20, Eli Zaretskii wrote:
>> Cc: drew.adams@oracle.com, 4911@debbugs.gnu.org, larsi@gnus.org
>> From: Clément Pit-Claudel <clement.pitclaudel@gmail.com>
>> Date: Fri, 8 May 2020 11:01:35 -0400
>>
>>> I'm not sure I understand what you mean by "span" in general and
>>> "current span" in particular.
>>
>> I meant a range of text with a single mouse-face property.
> 
> But if the underlying text has different face properties, the range of
> text should now be divided into different "spans", no?

Yes, sorry for the confusing terminology :'(
Basically I meant to say that we keep the code used for locating which span of 
text is covered by a mouse highlight; namely, we look for a range of text with 
an `eq' mouse-face value.

>>>   . realizing and caching 2 faces whenever we render some text which
>>>     has a mouse-face property;
>>
>> Don't we already do this, currently?
> 
> No, because we only have a single mouse-face.  With this feature, we'd
> have multiple ones, and they are only known when the face of the text
> is being realized, because each character could have different sources
> of face information, which need to be merged.

Ah, I see.  Right now we don't need to look at the regular faces at all; of 
course.

>>>   . using the corresponding face when redrawing the highlighted
>>>     portions of text by looking at the positions of each of the
>>>     affected glyphs (which might be complicated if the text doesn't
>>>     come from a buffer)
>>
>> But isn't that already taken care of by the existing highlighting code?
> 
> No, because the existing code always uses the same mouse-face.  So it
> only needs to know which glyphs need to be redrawn with that single
> face.

Got it.  My hope was that changing Mouse_HLInfo would allow us to get this 
almost "for free"

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