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bug#5718: scroll-margin in buffer with small line count.
From: |
npostavs |
Subject: |
bug#5718: scroll-margin in buffer with small line count. |
Date: |
Sat, 13 Aug 2016 18:01:43 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
Eli Zaretskii <eliz@gnu.org> writes:
>>
>> I think the complaint is that the `scroll-margin' effective value is
>> capped at a 1/4 of the window height, as seen in this
>> window_scroll_pixel_based (window.c):
>>
>> 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 was looking at fixing it, but I got confused by this comment in
window.h:
/* 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?
>
>> Whereas, it seems more logical to cap it at half window height.
>
> No, I think it would leave too few lines for moving the cursor. This
> has been Emacs behavior since time immemoriam, so if we want to have a
> different behavior, it should be implemented an opt-in option, not the
> default.
Yes, sure. This is about Emacs' behaviour after the user has customized
scroll-margin, so the default isn't in question anyway. I imagine
something like this:
DEFVAR_LISP ("minimum-non-scroll-lines", Vminimum_non_scroll_lines,
doc: /* Lines around window's center where `scoll-margin' doesn't apply.
If point is within this many lines from the window's center, it will
not cause scrolling regardless of the value of `scroll-margin'. If
this is a float then it represents a fraction of the current window's
lines. */);
Vminimum_non_scroll_lines = make_float (1.0/4.0);
And then setting this to 1 would make scroll-margin have the effect that
the OP expects.