Re: Redisplay resets vscroll when window start changes

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> >> Would there be a way to distinguish the case where line-move can return
>> >> a correct result without changing vscroll, from the case where it does
>> >> require changing vscroll in order to find its answer?
>> > I don't think I understand the question.
>> 
>> The question can be rephrased as: could `line-move` only reset vscroll
>> in the case where we're scrolling through a very tall image?
>
> It currently handles the case of a tall screen line, which could be
> due to an image, an xwidget, or simply very large font.
>
> If you want to limit it only to those cases, then perhaps the answer
> is yes, but the problem with those functions is that they try to
> handle many use cases that are not completely described anywhere, so
> testing whether a particular change could break any of them is nigh
> impossible in practice.  And the code does work for the use cases for
> which it was written.  Which is why I suggested a completely new
> command for the purpose under discussion.

I must be missing something: in the test case posted here, AFAICT we
just have a call to `previous-line` which doesn't involve any tall
screen line, yet the result is still that vscroll is reset.


        Stefan