bug#73752: 29.4; Ligatures are randomly rendered with extra spaces

[Yixuan Chen <xuan@xlk.me>]

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On 10/28/24 10:25, Eli Zaretskii wrote:
>> Date: Sun, 27 Oct 2024 16:32:34 -0400
>> Cc: visuweshm@gmail.com, luangruo@yahoo.com, 73752@debbugs.gnu.org
>> From: Yixuan Chen <xuan@xlk.me>
>>
>> On 10/27/24 16:07, Eli Zaretskii wrote:
>>> To convince me that this is really happening (although I'm unable to
>>> understand how it could, given how Emacs faces work), you will need to
>>> show some code which generates such a situation in a reproducible
>>> manner, and then show me by using "M-x describe-text-properties" and
>>> "C-u C-x =" that indeed the same characters in the same face are shown
>>> on different lines with different metrics.
>>
>> OK, here you go.
> 
> Not exactly what I asked for, or understood how the problem manifests
> itself...
> 
>> "screenshot1.png" shows the bugged display. Here's the result of
>> "describe-text-properties",
>>> There are text properties here:
>>>    face                 (face12 font-lock-string-face)
>>>    fontified            t
> 
> But this is a completely different issue.  There's no indentation
> here, right? You are saying that in the "bad" display there's some
> extra space between the ligature and the following quote, right?

Maybe my use of words were terrible. The problem I want to report was 
never about indentation. It was always about the "extra space between 
the ligature and the following character" (and sometimes also some extra 
space between the ligature and the character, although not in this 
instance).

> Is
> that extra space a real SPC glyph or is it just that the ligature is
> considered "wider"?  What happens if you put the cursor on the ▷
> ligature in the "bad" display -- does the block cursor then take up
> all the space up to the next quote?

It's not a real SPC glyph. It's a single character ▷ (or ligature/glyph 
I should call it? the underlying text is "|||>"). I attached five 
screenshots, where the block cursor is at ", |, |, |, >, " respectively. 
Noticeably, the block cursor for the first | character (1_bar.png) is 
extra wide, and the block cursors for the following characters looks 
offset from where they are.

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