bug#66948: [PATCH] Add Completion Preview mode

[Eshel Yaron]

Tags: patch

Following the recent discussion on emacs-devel[0], I'm attaching a patch
that adds a new Completion Preview minor mode.

I've been testing this patch with different completion backends, and so
far I'm pretty happy with the results.  I've made some tweaks compared
to the version I posted on emacs-devel to make the implementation more
robust, and in particular to accommodate for some peculiarities of
LSP-based completion, which I've tested mostly with `eglot` but also
with `lsp-mode`.

In terms of documentation, this patch extends "(emacs)Symbol Completion"
with a description of Completion Preview mode.  That seemed like a good
place to discuss this mode, but let me know if somewhere else would be
preferable.

Another question is whether to have RET/TAB/both accept and insert the
completion suggestion.  In VS Code, both RET and TAB do that AFAICT.  If
we want to, we can easily do that too by binding RET/TAB in
`completion-preview-active-mode-map`.  Alternatively, we can just
suggest doing so in the documentation.  The current patch binds neither
RET nor TAB, so to accept the suggestion you complete with C-M-i.
Personally, I find that convenient enough, but I can imagine that many
users might expect TAB or RET to work too.

[0] https://lists.gnu.org/archive/html/emacs-devel/2023-10/msg00693.html

0001-Add-Completion-Preview-mode.patch
Description: Text Data