bug#5718: scroll-margin in buffer with small line count.

[npostavs]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: npostavs@users.sourceforge.net
>> Cc: 5718@debbugs.gnu.org,  ahyatt@gmail.com,  gavenkoa@gmail.com
>> Date: Sat, 13 Aug 2016 18:01:43 -0400
>> 
>> >>   this_scroll_margin = max (0, scroll_margin);
>> >>   this_scroll_margin
>> >>     = min (this_scroll_margin, window_total_lines / 4);
>> >
>> > Which reveals a subtle bug: the actual scroll margin should be 1 for 7
>> > lines, 2 for 11, etc.  The problem is that the value of
>> > window_total_lines includes the mode line, which it shouldn't.  Maybe
>> > this should be fixed.

I have a patch set for fixing this and allowing the user to change the
maximum margin from 0.25.  The latter doesn't quite work perfectly, for
some reason when setting the maximum margin to 0.5 and scroll-margin to
100, `scroll-down-command' doesn't keep point centered in the window,
even though other commands (e.g. `scroll-up-command') do.  The patches
come with tests demonstrating this (the tests only work in interactive
mode).

v1-0001-Refactor-uses-of-scroll_margin-to-a-function.patch.gz
Description: compressed patch

v1-0002-Don-t-count-mode-line-for-scroll-margin-limit.patch
Description: patch

v1-0003-BROKEN-Make-limit-on-scroll-margin-variable.patch
Description: patch

>> 
>> 
>> /* Height in pixels, and in lines, of the mode line.
>>    May be zero if W doesn't have a mode line.  */
>> #define WINDOW_MODE_LINE_HEIGHT(W)   \
>> 
>> How is the height "in pixels, and in lines"?  Doesn't it have to be one
>> or the other?
>
> It's in pixels.  The comment should be fixed.

Fixed pushed as ea0f750e, "Fix comments on window height macros"