bug#56815: 29.0.50; Isearch lazy-highlight highlights too much when truncate-lines is in effect

[Gregory Heytings]

> > The problem is how "visible" is defined here (by 
> > pos-visible-in-window-p). With truncated lines, all buffer positions 
> > between window-start and window-end are "visible", or IOW, all buffer 
> > positions on the left or right of the actually visible line portion are 
> > "visible".
>
> One could use posn-at-point instead.

One could, but...

> > Moreover pos-visible-in-window-p is a slow function, so it would be 
> > terribly slow to call it for each candidate position in a buffer with 
> > (tens of) thousands of candidate positions.
>
> I think posn-at-point is much faster,

No, it is slower.  On my laptop, with long-line.xml and truncate-lines:

(benchmark-run 1000 (pos-visible-in-window-p 310000)) takes 4 seconds

(benchmark-run 1000 (numberp (cadr (posn-at-point 310000)))) takes 6 seconds