bug#24633: highlight-region func using (window-hscroll) in :align-to spec can cause inf loop

[npostavs@users.sourceforge.net]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: npostavs@users.sourceforge.net
>> Cc: 24633@debbugs.gnu.org
>> Date: Sat, 08 Oct 2016 13:17:37 -0400
>> 
>> The high-level idea is to have a line from going from the left to the
>> right edge of the window.  Most of the line is composed of an overlay
>> specifying the :underline text property.  The last bit of it, from the
>> end of the text to the right edge of the screen, is created by an
>> underlined after-string space which uses :align-to to reach the right
>> edge of the screen.
>> 
>> Using :align-to 'right or :align-to `((,(window-body-width nil t))) does
>> not account for horizontal scrolling (i.e., the line ends at 80 columns,
>> even when the 80th column is scrolled to the left).  So I tried
>> 
>>     `(+ ,(window-hscroll) (0 . right))
>>     `(+ ,(window-hscroll) (,(window-body-width nil t))) ; initially this
>> 
>> to make up for it, which turns out to cause this strange looping.
>
> Automatic hscrolling is based on length of point's line at the end of
> redisplaying a window.  Here, "line length" means the horizontal space
> required for displaying all the glyphs of the line, which includes the
> glyphs that come from the after-string.  Since you extend your
> after-string each time the window is hscrolled, you in effect enlarge
> the line each time the display engine hscrolls the window, which
> causes an endless loop.
>
> Can you restrict the after-string's length so it always ends at the
> last character of the line, or some fixed number of columns afterward?

Then it won't reach the edge of the screen when the buffer is scrolled.
I guess what I would expect is that the automatic hscrolling only goes
as far as the cursor is rendered, which should be the end of text where
I put the `cursor' property.  If you think this would be too
complex/slow to implement, we can mark this bug as wontfix.

>
>> >  In particular, I don't
>> > understand the align-to expression: e.g., window-hscroll returns its
>> > value in columns, while align-to needs pixels, AFAIU.
>> 
>> According to `(elisp) Pixel Specification',
>> 
>>        The form NUM specifies a fraction of the default frame font height
>>     or width.  The form `(NUM)' specifies an absolute number of pixels.
>
> I admire your courage in reading that documentation and then writing
> stuff like the above, which the documentation doesn't mention even
> remotely.

Uh, not sure how to read this, is it irony?