>> >>   (or truncate-lines (truncated-partial-width-window-p))
>> >
>> > Fine by me.
>> 
>> Then I'll push a better patch:
>
> Btw, I didn't use window-total-width instead of window-width.  What I
> did mimics what the display engine does when it determines whether to
> truncate lines in a window.  So I think
> truncated-partial-width-window-p should be fixed to use
> window-total-width instead.

Now this patch fixes truncated-partial-width-window-p to use
window-total-width.  Also it fixes the recent code for
long_line_optimizations_p in scan_for_column to better mimic
what the display engine does: