bug#56683: 29.0.50; long lines fix doesn't work correctly when lines are truncated

[Gregory Heytings <gregory@heytings.org>]

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>
> I'm using emacs with `truncate-lines` set to `t` in my init.el.  After 
> the merge of #56393 I've rebuilt Emacs, and started testing the fix. 
> I've noticed, that when lines are truncated, opening dictionary.json 
> from the #45898 and pressing C-e doesn't move you to the end of the 
> line, but stops somewhere inside it.  E.g. with the default window width 
> of Emacs -Q, I can see the following portion of the line
>
> ←, "hu": ["index"]}, "examples": [["en C la indicoj aperas interortajkrampoj, →
>
> Arrows shown in the fringe indicate that there are more to the line on 
> the right side.
>

You would have seen the same bug before the merge, so it's entirely 
unrelated to the merge.  It is present at least in Emacs 24, 25, 26, 27 
and 28.

>
> As a side question, does these optimizations work with truncated lines? 
> As far as I can tell Emacs is not as responsive as when the lines are 
> wrapped, but I don't know how to measure this precisely.
>

They do work with truncated lines, but I suspect that truncated lines 
require specific fixes to work efficiently.