> We currently don't visit the file buffer if it's not already visited,
> parsing the line in a temp buffer instead. That approach resulted in a nice
> perf improvement.

Reusing visited files is a nice feature, but still needs a fix.

Test case: visit emacs/src/xdisp.c and type

  C-x p g expose_frame RET

See that not all lines from xdisp.c have font-lock highlighting.
After applying this patch, all xref output lines from xdisp.c
have font-lock faces: