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

[Yixuan Chen <xuan@xlk.me>]

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On 10/28/24 11:05, Eli Zaretskii wrote:
>> Date: Mon, 28 Oct 2024 10:44:23 -0400
>> Cc: visuweshm@gmail.com, luangruo@yahoo.com, 73752@debbugs.gnu.org
>> From: Yixuan Chen <xuan@xlk.me>
>>
>>> 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.
> 
> According to the screenshots, it actually looks like it's a real SPC
> glyph or something.  What does "C-u C-x =" show when the block cursor
> is shown as below?

It shows
>              position: 1277 of 3527 (36%), column: 34
>             character: > (displayed as >) (codepoint 62, #o76, #x3e)
>               charset: ascii (ASCII (ISO646 IRV))
> code point in charset: 0x3E
>                script: latin
>                syntax: _ 	which means: symbol
>              category: .:Base, a:ASCII, l:Latin, r:Roman
>              to input: type "C-x 8 RET 3e" or "C-x 8 RET GREATER-THAN SIGN"
>           buffer code: #x3E
>             file code: #x3E (encoded by coding system utf-8-unix)
>               display: by this font (glyph code):
>     ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x810)
> 
> Character code properties: customize what to show
>   name: GREATER-THAN SIGN
>   general-category: Sm (Symbol, Math)
>   decomposition: (62) ('>')
> 
> There are text properties here:
>   face                 (face12 font-lock-string-face)
>   fontified            t

> Actually, how about showing what "C-u C-x =" says in each of the 6
> positions you show?  And then show what "C-u C-x =" says in the same 6
> positions in the "good" display?

See "bad.txt" for the output of the bad emacs display, and "good.txt" 
for the output of good emacs display. I also attached the block cursor 
screenshots for good emacs display.

[-- Attachment #2: good.txt --]
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0:

             position: 1273 of 3535 (36%), column: 30
            character: " (displayed as ") (codepoint 34, #o42, #x22)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x22
               script: latin
               syntax: " 	which means: string
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 22" or "C-x 8 RET QUOTATION MARK"
          buffer code: #x22
            file code: #x22 (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x779)

Character code properties: customize what to show
  name: QUOTATION MARK
  general-category: Po (Punctuation, Other)
  decomposition: (34) ('"')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

1:

             position: 1274 of 3535 (36%), column: 31
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

2:

             position: 1275 of 3535 (36%), column: 32
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

3:

             position: 1276 of 3535 (36%), column: 33
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

4:

             position: 1277 of 3535 (36%), column: 34
            character: > (displayed as >) (codepoint 62, #o76, #x3e)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x3E
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 3e" or "C-x 8 RET GREATER-THAN SIGN"
          buffer code: #x3E
            file code: #x3E (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x810)

Character code properties: customize what to show
  name: GREATER-THAN SIGN
  general-category: Sm (Symbol, Math)
  decomposition: (62) ('>')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

5:

             position: 1278 of 3535 (36%), column: 35
            character: " (displayed as ") (codepoint 34, #o42, #x22)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x22
               script: latin
               syntax: " 	which means: string
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 22" or "C-x 8 RET QUOTATION MARK"
          buffer code: #x22
            file code: #x22 (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x779)

Character code properties: customize what to show
  name: QUOTATION MARK
  general-category: Po (Punctuation, Other)
  decomposition: (34) ('"')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

[-- Attachment #3: bad.txt --]
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0:

             position: 1273 of 3527 (36%), column: 30
            character: " (displayed as ") (codepoint 34, #o42, #x22)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x22
               script: latin
               syntax: " 	which means: string
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 22" or "C-x 8 RET QUOTATION MARK"
          buffer code: #x22
            file code: #x22 (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x779)

Character code properties: customize what to show
  name: QUOTATION MARK
  general-category: Po (Punctuation, Other)
  decomposition: (34) ('"')

1:

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

             position: 1274 of 3527 (36%), column: 31
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

2:

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

             position: 1274 of 3527 (36%), column: 31
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

3:

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t
             position: 1276 of 3527 (36%), column: 33
            character: | (displayed as |) (codepoint 124, #o174, #x7c)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x7C
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 7c" or "C-x 8 RET VERTICAL LINE"
          buffer code: #x7C
            file code: #x7C (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x9F6)

Character code properties: customize what to show
  name: VERTICAL LINE
  old-name: VERTICAL BAR
  general-category: Sm (Symbol, Math)
  decomposition: (124) ('|')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

4:

             position: 1277 of 3527 (36%), column: 34
            character: > (displayed as >) (codepoint 62, #o76, #x3e)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x3E
               script: latin
               syntax: _ 	which means: symbol
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 3e" or "C-x 8 RET GREATER-THAN SIGN"
          buffer code: #x3E
            file code: #x3E (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x810)

Character code properties: customize what to show
  name: GREATER-THAN SIGN
  general-category: Sm (Symbol, Math)
  decomposition: (62) ('>')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

5:

             position: 1278 of 3527 (36%), column: 35
            character: " (displayed as ") (codepoint 34, #o42, #x22)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x22
               script: latin
               syntax: " 	which means: string
             category: .:Base, a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 22" or "C-x 8 RET QUOTATION MARK"
          buffer code: #x22
            file code: #x22 (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    ftcrhb:-SAJA-Cascadia Code-light-normal-normal-*-17-*-*-*-m-0-iso10646-1 (#x779)

Character code properties: customize what to show
  name: QUOTATION MARK
  general-category: Po (Punctuation, Other)
  decomposition: (34) ('"')

There are text properties here:
  face                 (face12 font-lock-string-face)
  fontified            t

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