Re: The unwarranted scrolling assumption

[Lennart Borgman]

Maybe I should have sent this to you before. I saved it as a draft
(this is part of it), but thought it was unnecessary when I later
understood the root of the problem.

This is what I saw (with some debug tracing):

- The path try_scrolling is not taken.
- This is because clip_changed is 1.
- narrow_to_region set it to 1.
- reconsider_clip_changes gets a chance to reset it to 0, but it does
not in this case.
- reconsider_clip_changes does not however set it to 1.

The reason that reconsider_clip_changes does not set clip_changed to 0 is that

   !NILP (w->window_end_valid),

which looks like window_end_valid is non-nil.

This is and the code logic itself is the basis for my patches.