bug#62048: 30.0.50; Non-nil `line-spacing' takes precendence over 'line-height t text property

[Ihor Radchenko <yantar92@posteo.net>]

Eli Zaretskii <eliz@gnu.org> writes:

>> Observed: top of the image is displayed
>> Expected: bottom of the image is partially revealed
>
> I don't understand the expectation.  Scrolling by vscroll only happens
> when before the scroll some part of the image is visible, which is not
> what happens here.
>
> If you want to change that, feel free to hack on the code in
> simple.el, but there was no intent to cover this particular use case,
> and the code is already quite complicated (to say the least).
> ...
> You should always keep in mind that Emacs has no idea about what's
> beyond the window, for display purposes.  There's no way of saying
> whether a given 'display property whose value is an image spec will be
> taller than the window, except by actually displaying that image (or
> at least simulating the display internally).  So you expect something
> that it is far from easy to do.
>
> Scrolling commands were never meant to allow smooth scrolling through
> tall screen lines.

What if the code instead tries to vscroll standard line height first and
only then decide if we want to scroll further, displaying tall line?

> What do you want to customize, and in what terms?

I was referring to

>> 	      (if (and (< arg 0)
>> 		       (< (point) (window-start))
>> 		       (> lh winh))
>> 		  (set-window-vscroll
>> 		   nil
>> 		   (- lh dlh) t)))

May we allow users to customize ~(> lh winh)~ condition?
By providing a custom setting that will determine the threshold for
partial scroll: `line-move-height-threshold'. The default value will be
'window making lines taller than window height to trigger partial
scroll. Alternative values could be a subset of pixel specs, like (NUM)
or (X . UNIT).

Also, on the initial report. Is it possible to increase default
buffer-wise line-spacing via text property? (AFAIU, decreasing is
impossible).

-- 
Ihor Radchenko // yantar92,
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