Subject: 25.1.50; window-state-put, image-mode and window scrolling
From: Andreas Politz <[email-masked]>
To: [email-masked]
Date: Tue, 16 Aug 2016 01:05:11 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

There is a conflict between window-state-put and image-mode and
maybe other modes.  

The function image-mode-reapply-winprops gets called as soon as
window-state-put displays the image buffer in some window via
window-configuration-change-hook and applies the image's
previously stored scroll-values.

But window-state-put is not done yet and it has it's own
understanding of how the window should be scrolled, thereby
overriding images-mode's scrolling.

I guess ideally window-state-put should be atomic in the sense
that it delays these kinds of hooks until the window is
completely restored.  Below is a recipe and I attached a
workaround.

-ap

--

Let's start with

emacs -Q

and find some image file:

C-x C-f foo.png RET

Maybe enlarge the image, such that it can be scrolled.  Move the
image (C-n), such that vscroll is greater 0.

(window-vscroll) => 37

Save the window state,

(setq wst (window-state-get))

and switch to the *scratch* buffer.

(current-buffer) => #<buffer *scratch*>

Now restore the previously saved window state

(window-state-put wst)

,which should bring back the image buffer.  But notice, how
vscroll seems to be 0, i.e. display starts at the top of image.
Finally evaluate any expression in the mini-buffer

M-: t RET

which triggers image-mode-reapply-winprops scrolling the image as
expected.