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

[Visuwesh <visuweshm@gmail.com>]

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[புதன் அக்டோபர் 30, 2024] Eli Zaretskii wrote:

>> From: Tim Ruffing <dev@real-or-random.org>
>> Cc: dev@real-or-random.org, eggert@cs.ucla.edu, 73752@debbugs.gnu.org
>> Date: Wed, 30 Oct 2024 18:34:14 +0100
>> 
>> 
>> > > pp is calling Emacs to print to its stderr.  If that is redirected
>> > > somewhere else you won't see the output.
>> > 
>> 
>> Ah, this was the right hint. I'm using emacs in daemon mode, started
>> from systemd, so I can inspect stderr via journalctl.
>> 
>> ------
>> 
>> Broken rendering for ligature "===" starting at pos 1290 (emacs happens
>> to be in daemon mode)
>> 
>> [...]
>>  39  480: COMP[19 (0..0)] pos=1290 w=20 a+d=20+6 face=39 MB
>>  40  500: COMP[19 (1..1)] pos=1291 w=20 a+d=20+6 face=39 MB
>>  41  520: COMP[19 (2..2)] pos=1292 w=20 a+d=20+6 face=39 MB
>> [...]
>> 
>> $ pp composition_gstring_from_id(19)
>>  
>>  [[#<font-object "-UKWN-Iosevka Term SS04-regular-normal-normal-*-20-*-*-*-d-0-iso10646-1"> 61 61 61] 19 [0 0 61 5852 10 1 11 11 -4 [0 0 20]] [1 1 61 5896 10 -1 11 11 -4 [0 0 20]] [2 2 61 5891 10 -1 9 11 -4 [0 0 20]]]
>> 
>> -------
>> 
>> Proper rendering (emacs happens to be not in daemon mode):
>> 
>>  39  390: COMP[13 (0..0)] pos=1290 w=10 a+d=20+6 face=39 MB
>>  40  400: COMP[13 (1..1)] pos=1291 w=10 a+d=20+6 face=39 MB
>>  41  410: COMP[13 (2..2)] pos=1292 w=10 a+d=20+6 face=39 MB
>> 
>> $ pp composition_gstring_from_id(13) 
>> 
>>  [[#<font-object "-UKWN-Iosevka Term SS04-regular-normal-normal-*-20-*-*-*-d-0-iso10646-1"> 61 61 61] 13 [0 0 61 5852 10 1 11 11 -4 nil] [1 1 61 5896 10 -1 11 11 -4 nil] [2 2 61 5891 10 -1 9 11 -4 nil]] 
>> 
>> 
>> Both sessions are still running. I Hope this helps. Let me know if need
>> more remote hands.

I observe very similar results:

Properly rendered (in a fresh Emacs session):

    [[#<font-object "-SAJA-Cascadia Code-bold-normal-normal-*-15-*-*-*-m-0-iso10646-1"> 45 45 62] 2 [0 0 45 1970 9 0 9 7 -4 nil] [1 1 45 1969 9 -1 10 7 -4 nil] [2 2 62 2728 9 -1 9 11 0 nil]]

Misaligned:

    [[#<font-object "-SAJA-Cascadia Code-bold-normal-normal-*-15-*-*-*-m-0-iso10646-1"> 45 45 62] 2231 [0 0 45 1970 9 0 9 7 -4 [0 0 10]] [1 1 45 1969 9 -1 10 7 -4 [0 0 10]] [2 2 62 2728 9 -1 9 11 0 [0 0 10]]]

In the misaligned session, cached entry for the same text ("-->") that
is rendered properly at a different font size:

    [[#<font-object "-SAJA-Cascadia Code-bold-normal-normal-*-17-*-*-*-m-0-iso10646-1"> 45 45 62] 2179 [0 0 45 1970 10 0 11 7 -4 nil] [1 1 45 1969 10 -1 11 7 -4 nil] [2 2 62 2728 10 -1 10 12 0 nil]]



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> Did the "bad" display start from "good" at the beginning of a session?
> Or did it start from "bad" to begin with?  If the former, the next
> idea is to put a watchpoint on the cached composition in a session
> with "good" display, and then do whatever it takes to make it "bad",
> hoping that the watchpoint will break at some point and show us the
> code which replaces nil with these [X-OFF Y-OFF WADJUST] vectors.

I think it starts from "bad" to begin with.  But the former theory could
still apply, if you do not mind this vague answer, can you provide
instructions to set a watchpoint?  If the watchpoint never triggers, we
might be able to at least rule out the former theory.