> So this needs the buffer text be displayed in some window, before it
> could be printed?
>
Yes. Plain text file would not have any font/ display information otherwise.

> It uses following API to determine BIDI classification and processes
> accordingly:
> > (get-char-code-property (char-after from) 'bidi-class)
>
> Hmm... not sure this is enough, except in the simplest cases.  The
> bidi-class of a character is just its basic attribute; application of
> the full Unicode Bidirectional Algorithm can and does change the
> effective attributes of some characters, before they are displayed.
>
Do you have some sample text for this use case? I can run a quick test.

Is this test performed on each character or only at the beginning of a
> visual line?
>
This happens at every point where the font changes.

Apologies if I didn't understand what the code does correctly; I only
> had a cursory glance on it, and there are not so many comments
> explaining what it does.
>
No worries. I'm happy to answer any queries.