Re: Scrolling jumpy when line-spacing > 0

[Yuri Khan <yuri.v.khan@gmail.com>]

On Sat, Apr 22, 2017 at 1:56 AM, Eli Zaretskii <eliz@gnu.org> wrote:

>> > (Why do people report bugs here, rather than with report-emacs-bug?)
>>
>> To check if that is indeed a bug worth reporting, or if it is already
>> known, or even already fixed?
>
> All of these are good enough reasons to post to the bug tracker.

Okay.

>> As a user, I’d expect that additional spacing does not count toward
>> whether the line is considered visible.
>
> The Emacs display engine was designed to avoid having point in a
> partially visible line, for whatever reasons.  The code which checks
> for partial visibility doesn't care what is in the invisible part,
> because checking for that would take non-trivial processing, and the
> subtlety isn't important enough to slow down redisplay.

How come Emacs 24 lets me put point in that partially fully visible
line? Was something simplified between 24 and master?

To me, that kind of subtlety is important. Not that important to cry
regression, but important enough to wonder if I could afford the
slowdown. I’m even mildly bothered by the fact that spacing is
conceptually below its line; I’d find it nicer if half of it was
above.

>> The other special case is when the point is in the topmost visible
>> line.
>> […]
>> I see no practical way to detect this condition without moving point.
>
> Doesn't posn-at-point allow you to find out whether point is in the
> topmost window line?  (Caveat: this could require special
> consideration when there's a non-nil header-line-format in the
> window.)

Maybe. Maybe not. On the one hand, I do have a non-nil
header-line-format at all times. (For tabbar-mode.) On the other hand,
when I evaluate (posn-at-point) with point in the topmost window line,
I get zero Y and ROW values. It is not immediately clear if I can rely
on that.

On the third hand, using motion relative to the point is immediately
clear. What’s the downside? Is it going to be slow? Slow enough to be
noticeable at a key repeat rate of 40 Hz?