bug#71419: 30.0.50; Completion does not always highlight the "common part" corresponding to suffix

[Dmitry Gutov <dmitry@gutov.dev>]

X-Debbugs-Cc: monnier@IRO.UMontreal.CA

For example:

1. Type

   (fo|-function

where | denotes the position of point.

2. C-M-i

You get 5 completions (the exact number is not important), where "fo" is
highlighted in all of them, but "function is not.

3. Move point to after "-" (meaning, just one character forward), press
C-M-i again (twice).

Completion expands the text to "(fo-|-function" (the addition might be
unimportant) and "function" is highlighted with the face
`completions-common-part' now, in all completions.

This seems inconsistent.

Also, highlighting is information. If it was more uniform, we could use
it to e.g. address the FIXME in completion-all-completions. Though
that's up for discussion, given that highlighting is inherently less
reliable than other methods we use. But it would be a non-breaking
change, OT2H.

WDYT?

In GNU Emacs 30.0.50 (build 28, x86_64-pc-linux-gnu, X toolkit, cairo
  version 1.18.0, Xaw scroll bars) of 2024-06-07 built on potemkin
Repository revision: b571c6571c8bc4c34569650104aee273c19cbfd4
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12302000
System Description: Ubuntu 23.10