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

[Visuwesh]

[செவ்வாய் அக்டோபர் 29, 2024] Eli Zaretskii wrote:

>> 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.

Thank you very much for the clear instructions.  I was testing this in a
fresh Emacs session.  And

    (gdb) pp composition_gstring_from_id(ID)

seems to show nothing?

    (gdb) c
    Continuing.

    Thread 1 "emacs" hit Breakpoint 3, set_cursor_from_row (w=0x55b8c994f338, 
row=0x55b8c9e20410, matrix=0x55b8c9960480, delta=0, delta_bytes=0, dy=0, 
dvpos=0) at xdisp.c:18217
    18217         struct glyph *glyph = row->glyphs[TEXT_AREA];
    (gdb) pgrow
    TEXT: 4 glyphs
      0    0: COMP[16 (0..0)] pos=5 w=9 a+d=14+4 face=28 MB
      1    9: COMP[16 (1..1)] pos=6 w=9 a+d=14+4 face=28 MB
      2   18: COMP[16 (2..2)] pos=7 w=9 a+d=14+4 face=28 MB
      3   27: CHAR[ ] pos=0 blev=0,btyp=B w=9 a+d=14+4 MB
    (gdb) pp composition_gstring_from_id(16)
    (gdb) p composition_gstring_from_id(16)
    $1 = XIL(0x55b8cada607d)

Am I missing something?