Re: Need `truncate-string-to-pixel-width` and `glyph-pixel-width` functions in C

[Jimmy Yuen Ho Wong <wyuenho@gmail.com>]

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>
> I guess I don't understand the issue you are describing.  Where does
> window system come into play here?  What are "UI elements that Emacs
> thinks are content", and where are they in the screenshots shown in
> https://github.com/minad/corfu/pull/508 ?  And where in my suggestion
> did I propose anything that would cause "truncation by window system"?
>

That thin white bar on the right of the box from the third screenshot
onwards is actually a string with a display space property in pixels, not a
real scroll bar.

Using :align-to in the padding between the candidate and annotation is not
a problem, the problem is if the concatenated line is too long, I'll need
to truncate. There are 2 options - `truncate-string-to-width`, which is
bugged, or set the `truncate-line` buffer local to t and let Emacs' window
system do its magic, which will be performant and correct, and I can even
replace the arrow in an ellipsis, but truncation will start to truncate off
the emulated scroll bar.


> Lisp programs are not supposed to do layout calculations, plain and
> simple.  The reason is that layout calculations are impossible without
> having a window with lots of stuff that determines how text is
> displayed.  So doing that on strings is meaningless.
>

As I described in my first email, this can almost be achieved from the
existing Elisp functions and C functions exposed Elisp. The performance
problem associated with resolving a font WRT the anonymous face and face
list craziness can be done in C, and already exists. The width of the
display space and string replacement text properties can be calculated by
`check_display_width` in `indent.c`, and rest is surely somewhere in
`image.c`. I hope you are not telling me that Emacs devs still refuse to
give people the ability to align text in pixel precisions programmatically
at the end of 2024. Browsers have had this ability for almost 30 years.

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