bug#48734: 28.0.50; Performance regression in `string-width`?

[Eli Zaretskii <eliz@gnu.org>]

> From: Imran Khan <contact@imrankhan.live>
> Cc: 48734@debbugs.gnu.org
> Date: Sun, 30 May 2021 17:23:15 +0600
> 
> Basically deft-mode takes a folder full of normal org-mode files, and
> constructs a pretty "dashboard" view of the folder where you can browse,
> search, filter the files in said folder (and many other features).
> 
> The dashboard UI uses `string-width` to calculate how much space is to
> be allocated relative to window width to display metadata like file
> title, file content summary, mtime etc for each file per line. This is
> dynamic, the components size adjusts to window width change. Perhaps the
> screenshot they have in their project page would be more descriptive:
> 
> https://github.com/jrblevin/deft
> 
> Org-mode files typically have unicode chars in them. So when deft-mode
> uses `string-width` to construct view of file content part, it
> hangs. I think the performance problem here is exacerbated because
> deft-mode is stripping all vertical whitespace to squash the content to
> show in a single line summary view, before calling `string-width`.

If deft-mode wants to allocate space on display, then they really do
need to use string-width, but then the changes which make them "hang"
are really important, because before that string-width would compute
the result incorrectly when characters are composed on display.