Possible bug in \definefontfallback with check=yes

Hi Hans,

While testing Semitic languages for multilingual bibliography support
(in this case Aramaic/Syriac), I came across a reproducible problem with
`check=yes` in `\definefontfallback`.

The tests were made with:

     ConTeXt 2026.07.29 11:46 LMTX
     LuaMetaTeX 2.11.09
     functionality level 20260727

The issue first appeared in a bibliography test using Libertinus Serif as
the main text font and Estrangelo Edessa as a Syriac fallback. I then
reduced it to the font fallback mechanism itself, without BTX/SBL or bidi.

I also repeated the test with SBL BibLit as the main font in order to rule
out a Libertinus-specific interaction. The same result occurs with both
main fonts:

                    check=yes,force=yes        force=yes
                          ------------------- ---------
Libertinus Serif             FAIL                   WORKS
SBL BibLit                      FAIL                    WORKS

In all cases the Edessa fallback font is found and loaded. With
`check=yes`, however, the Syriac characters are not taken from the
fallback font. Removing only `check=yes` makes the fallback work.

I found an earlier related report by Alan Bowen on the list in 2021
("rtl fontfallback family issue?"), also involving Syriac, Estrangelo
Edessa, and font fallback under LMTX. That example used
`\definefallbackfamily` and seems to concern a different issue, but I
mention it because of the closely related Syriac/fallback context.

Here is a MWE using SBL BibLit:

------------------------------------------------------------

\definefontfallback
   [testfallback]
[file:/usr/share/fonts/opentype/xfree86-nonfree-syriac/SyrCOMEdessa.otf]
   [0x0700-0x074F]
   [check=yes,force=yes]

\definefontsynonym
   [TestSerif]
   [file:SBL_BLit.ttf]
   [fallbacks=testfallback]

\definefont
   [TestFont]
   [TestSerif at 12pt]

\starttext

\TestFont

Latin: Test

Greek: λόγος

Hebrew: שלום

Syriac: ܟܬܒܐ

\stoptext

------------------------------------------------------------

With

     [check=yes,force=yes]

SBL BibLit and SyrCOMEdessa are both loaded, but the Syriac characters
are reported as missing from SBL BibLit and are not rendered.

If I change only

     [check=yes,force=yes]

to

     [force=yes]

the Syriac fallback is used and the characters are rendered.

The same A/B behaviour is reproducible with Libertinus Serif as the main
font.

This led me to look at `font-col.lmt`, in
`collections.clonevector()`.

In the `check` branch, when `target` is not set, I find:

     elseif check then
         ...
         else
             for unicode = start, stop do
                 local unic = unicode + offset - start
                 if isprivate(unic) or isprivate(unicode) then
                     -- ignore
                 elseif not newchars[target] then
                     -- not in font
                 elseif force or (not vector[unic] and not 
oldchars[unic]) then
                     vector[unic] = cloneid
                     ...
                 end
             end

This is the `else` branch of

     if target then
         ...
     else
         ...
     end

so `target` appears to be nil at this point.

Consequently,

     newchars[target]

does not seem to test whether the Unicode character currently being
processed is present in the fallback font.

This matches the observed behaviour closely: when `check=yes` is active,
no useful fallback vector is produced for the Syriac range; when
`check=yes` is removed, the same font, range, and fallback work.

Would the test here perhaps need to use

     newchars[unicode]

or

     newchars[unic]

depending on the intended handling of `offset`?

I first encountered this while testing Syriac font coverage for
multilingual bibliography support, but the reduced example above is
independent of the bibliography and bidi mechanisms.

Best,
JP

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