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

[Gregory Heytings]

> > Do I understand correctly that it's okay to do that on the feature (and 
> > later master) branch, and that it will be perhaps be revisited later?
>
> I'd prefer not to do it just yet, and wait for more user feedback (once 
> the feature branch is merged, which I guess will be soon).

Wouldn't it be easier to get useful user feedback if that automatic 
disabling was present on master?  If it's absent, what kind of user 
feedback can we expect to decide what is better for Emacs 29?

> > But still, as you say, that puts a limit at around 100 MB, which isn't 
> > "that much" nowadays.
>
> Yes, it's not a solution, just a band-aid.  But it costs almost nothing, 
> and we get to push the problematic situations farther.

It does indeed, but alas the fact that displaying such buffers is 
noticeably slower with truncate-lines remains.  I could perhaps take a 
look (after finalizing and merging the current branch), but I'm not really 
sure it's worth the price.