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

[Eli Zaretskii]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: luangruo@yahoo.com,  73752@debbugs.gnu.org,  xuan@xlk.me
> Date: Tue, 29 Oct 2024 19:24:26 +0530
> 
> I seem to have run into the issue.  The attached images
> "cascadia-code-bold-15-good" and "-bad.png" are the desired and
> misaligned composite text of "-->" rendered in Cascadia Code bold 15
> font.  The same text is composed fine with Cascadia Code bold 17.

I'm not sure this is the same issue we are talking about, but here are
the instructions anyway:

 . Move the cursor to where this ligature is displayed
 . Use "C-x =" to display the buffer position of the ligature
 . Attaching GDB to Emacs, then type these commands:

    (gdb) source /path/to/emacs/src/.gdbinit
    (gdb) thread 1
    (gdb) break set_cursor_from_row
    (gdb) continue

The breakpoint in set_cursor_from_row will break soon enough.  The
first time it breaks, just type "continue".  Second time it breaks,
type:

    (gdb) pgrow

This will show you the entire screen line (a.k.a. "glyph row") where
the cursor is displayed, which is also the screen line of the
ligature.  Here's how a similar line with ligatures looks here:

  (gdb) pgrow
  TEXT: 80 glyphs
    0    0: CHAR[ ] pos=857 blev=0,btyp=L w=9 a+d=17+4 MB
    1    9: CHAR[ ] pos=858 blev=0,btyp=L w=9 a+d=17+4 MB
    2   18: CHAR[ ] pos=859 blev=0,btyp=L w=9 a+d=17+4 MB
    3   27: CHAR[ ] pos=860 blev=0,btyp=L w=9 a+d=17+4 MB
    4   36: CHAR[ ] pos=861 blev=0,btyp=L w=9 a+d=17+4 MB
    5   45: CHAR[ ] pos=862 blev=0,btyp=L w=9 a+d=17+4 MB
    6   54: CHAR[ ] pos=863 blev=0,btyp=L w=9 a+d=17+4 MB
    7   63: CHAR[ ] pos=864 blev=0,btyp=L w=9 a+d=17+4 MB
    8   72: CHAR[ ] pos=865 blev=0,btyp=L w=9 a+d=17+4 MB
    9   81: CHAR[ ] pos=866 blev=0,btyp=L w=9 a+d=17+4 MB
   10   90: CHAR[ ] pos=867 blev=0,btyp=L w=9 a+d=17+4 MB
   11   99: CHAR[ ] pos=868 blev=0,btyp=L w=9 a+d=17+4 MB
   12  108: CHAR[ ] pos=869 blev=0,btyp=L w=9 a+d=17+4 MB
   13  117: CHAR[ ] pos=870 blev=0,btyp=L w=9 a+d=17+4 MB
   14  126: CHAR[ ] pos=871 blev=0,btyp=L w=9 a+d=17+4 MB
   15  135: CHAR[ ] pos=872 blev=0,btyp=L w=9 a+d=17+4 MB
   16  144: CHAR[ ] pos=873 blev=0,btyp=L w=9 a+d=17+4 MB
   17  153: CHAR[ ] pos=874 blev=0,btyp=L w=9 a+d=17+4 MB
   18  162: CHAR[ ] pos=875 blev=0,btyp=L w=9 a+d=17+4 MB
   19  171: CHAR[ ] pos=876 blev=0,btyp=L w=9 a+d=17+4 MB
   20  180: CHAR[ ] pos=877 blev=0,btyp=L w=9 a+d=17+4 MB
   21  189: CHAR[ ] pos=878 blev=0,btyp=L w=9 a+d=17+4 MB
   22  198: CHAR[ ] pos=879 blev=0,btyp=L w=9 a+d=17+4 MB
   23  207: CHAR[ ] pos=880 blev=0,btyp=L w=9 a+d=17+4 MB
   24  216: CHAR[ ] pos=881 blev=0,btyp=L w=9 a+d=17+4 MB
   25  225: CHAR[ ] pos=882 blev=0,btyp=L w=9 a+d=17+4 MB
   26  234: CHAR[ ] pos=883 blev=0,btyp=L w=9 a+d=17+4 MB
   27  243: CHAR[ ] pos=884 blev=0,btyp=L w=9 a+d=17+4 MB
   28  252: CHAR[ ] pos=885 blev=0,btyp=L w=9 a+d=17+4 MB
   29  261: CHAR[ ] pos=886 blev=0,btyp=L w=9 a+d=17+4 MB
   30  270: CHAR["] pos=887 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   31  279: COMP[69 (0..0)] pos=888 w=9 a+d=17+4 face=24 MB
   32  288: COMP[69 (1..1)] pos=889 w=9 a+d=17+4 face=24 MB
   33  297: COMP[69 (2..2)] pos=890 w=9 a+d=17+4 face=24 MB
   34  306: CHAR["] pos=891 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   35  315: CHAR[ ] pos=892 blev=0,btyp=L w=9 a+d=17+4 MB
   36  324: CHAR["] pos=893 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   37  333: COMP[70 (0..0)] pos=894 w=9 a+d=17+4 face=24 MB
   38  342: COMP[70 (1..1)] pos=895 w=9 a+d=17+4 face=24 MB
   39  351: CHAR["] pos=896 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   40  360: CHAR[ ] pos=897 blev=0,btyp=L w=9 a+d=17+4 MB
   41  369: CHAR["] pos=898 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   42  378: COMP[71 (0..0)] pos=899 w=9 a+d=17+4 face=24 MB
   43  387: COMP[71 (1..1)] pos=900 w=9 a+d=17+4 face=24 MB
   44  396: COMP[71 (2..2)] pos=901 w=9 a+d=17+4 face=24 MB
   45  405: CHAR["] pos=902 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   46  414: CHAR[ ] pos=903 blev=0,btyp=L w=9 a+d=17+4 MB
   47  423: CHAR["] pos=904 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   48  432: COMP[72 (0..0)] pos=905 w=9 a+d=17+4 face=24 MB
   49  441: COMP[72 (1..1)] pos=906 w=9 a+d=17+4 face=24 MB
   50  450: COMP[72 (2..2)] pos=907 w=9 a+d=17+4 face=24 MB
   51  459: CHAR["] pos=908 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   52  468: CHAR[ ] pos=909 blev=0,btyp=L w=9 a+d=17+4 MB
   53  477: CHAR["] pos=910 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   54  486: COMP[73 (0..0)] pos=911 w=9 a+d=17+4 face=24 MB
   55  495: COMP[73 (1..1)] pos=912 w=9 a+d=17+4 face=24 MB
   56  504: CHAR["] pos=913 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   57  513: CHAR[ ] pos=914 blev=0,btyp=L w=9 a+d=17+4 MB
   58  522: CHAR["] pos=915 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   59  531: COMP[74 (0..0)] pos=916 w=9 a+d=17+4 face=24 MB
   60  540: COMP[74 (1..1)] pos=917 w=9 a+d=17+4 face=24 MB
   61  549: COMP[74 (2..2)] pos=918 w=9 a+d=17+4 face=24 MB
   62  558: CHAR["] pos=919 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   63  567: CHAR[ ] pos=920 blev=0,btyp=L w=9 a+d=17+4 MB
   64  576: CHAR["] pos=921 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   65  585: COMP[75 (0..0)] pos=922 w=9 a+d=17+4 face=24 MB
   66  594: COMP[75 (1..1)] pos=923 w=9 a+d=17+4 face=24 MB
   67  603: COMP[75 (2..2)] pos=924 w=9 a+d=17+4 face=24 MB
   68  612: CHAR["] pos=925 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   69  621: CHAR[ ] pos=926 blev=0,btyp=L w=9 a+d=17+4 MB
   70  630: CHAR["] pos=927 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   71  639: COMP[76 (0..0)] pos=928 w=9 a+d=17+4 face=24 MB
   72  648: COMP[76 (1..1)] pos=929 w=9 a+d=17+4 face=24 MB
   73  657: COMP[76 (2..2)] pos=930 w=9 a+d=17+4 face=24 MB
   74  666: CHAR["] pos=931 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   75  675: CHAR[ ] pos=932 blev=0,btyp=L w=9 a+d=17+4 MB
   76  684: CHAR["] pos=933 blev=0,btyp=L w=9 a+d=17+4 face=23 MB
   77  693: COMP[77 (0..0)] pos=934 w=9 a+d=17+4 face=24 MB
   78  702: COMP[77 (1..1)] pos=935 w=9 a+d=17+4 face=24 MB
   79  711: COMP[77 (2..2)] pos=936 w=9 a+d=17+4 face=24 MB

Each line here describes a glyph on display.  Where it says "CHAR[X]",
that's a character glyph of character X.  Where it says
"COMP[n (i..j)]", that's a composition whose cached ID is n, and i and
j are the characters in the composed sequence represented by this
glyph.

The "pos=NNNN" part is the buffer position from where each glyph came.

Find the cache ID of the composition which shows the problematic
ligature by its buffer position which you displayed at the beginning.
Let's assume that the ID of that composition is 69 (from the glyph
row shown above).  Then type:

   (gdb) pp composition_gstring_from_id(69)

This will show the composition cached at slot 116.  The structure of
the composition is described in the doc string of
composition-get-gstring.  Here's what I get here for the ligature
cached at slot 69:

    (gdb) pp composition_gstring_from_id(69)
    [[#<font-object "-outline-Cascadia 
Code-regular-normal-normal-mono-16-*-*-*-c-*-iso8859-1"> 45 45 62] 69 [0 0 45 
1970 9 1 10 17 4 nil] [1 1 45 1969 9 0 10 17 4 nil] [2 2 62 2728 9 0 9 17 4 
nil]]

It clearly shows the font used to display the 3 glyphs of this
ligature and the data of the 3 glyphs themselves.

The idea is then to compare what you get from the "bad" display with
what you get for the same ligature in a fresh Emacs session, where the
display should be good.

Let me know if you need more help or more detailed instructions or
have questions.

Thanks.