Re: How to scroll images in Emacs?

[N. Jackson]

At 01:04 -0400 on Monday 2014-02-17, Eli Zaretskii wrote:

>> From: nljlistbox2@gmail.com (N. Jackson)
>> Date: Sun, 16 Feb 2014 18:49:58 -0400
>> 
>> I'm not used to having images in Emacs, and I'm having difficulty
>> understanding their scrolling behaviour.
>> 
>> I can't seem to scroll them horizontally at all, and when I scroll them
>> vertically, as soon as the top edge of the image hits the top of the
>> window, the whole image jumps up completely out of view. This makes it
>> very hard to veiw the bottom portion of an image, although sometimes I
>> can sneak up on it from below. Often, however, the window then scrolls
>> back up to the top of the image.
>> [...]
>> Using GNU Emacs 24.3.50 of 2014-02-10.
>
> These problems shouldn't exist in the development version.  With
> yesterday's trunk, I can scroll images both horizontally and
> vertically, using just the arrow keys, and there are no jumps that you
> describe.
>
> If it doesn't work for you in "emacs -Q", please submit a bug report
> using "M-x report-emacs-bug RET", starting with "emacs -Q", and
> include the image that gives you this trouble.
>
> Thanks.

I am now running GNU Emacs 24.3.50 (x86_64-unknown-linux-gnu, GTK+
Version 3.8.8) of 2014-02-19 Repository revision: 116484
lekktu@gmail.com-20140219210406-y2s7lx244ojfl5on.

I am still having several "issues", some of which are clearly bugs, but
some of which might simply be that I don't understand the intended
behaviour. Should I report separate bugs for Image mode, Eww, Gnus,
and the frame resizing?

A. An image in Image mode
=========================

emacs -Q
C-x C-f ~/my-image.jpg

Note: This test image is larger than the display I am using.

1. The top left of image image is displayed. Scrolling with arrow keys
works properly and smoothly left/right up/down. But scroll bars don't
work. Vertical scroll bar jumps back to top whenever it's pulled
down. Horizontal scroll bar is missing entirely from window.

2. There is no menu option for resizing image to the window. Is there a
command for this?

3. M-x image-transform-fit-to-width has no effect, but no error is
written to the *Messages* buffer. (Note: My build includes ImageMagick
support. Is this perhaps disabled with "emacs -Q"?)

4. The menu option "Fit Frame to Image" does not work properly. The
frame increases in size to _almost_ fill the display, but there is a gap
of a few pixels along the left side and along the bottom, and the top of
the window is about one title-bar-width below the top of the
display. Furthermore, the system doesn't seem to know how big the window
is, because if I now maximise the frame (with the logo key and the up
arrow) and restore it again (with the logo key and the down arrow), it
"restores" to another size that is smaller than the size after the "Fit
Frame to Image" command, but not nearly as small as the original size
when Emacs started. (I think I read a bug report a month or two ago of
similar behaviour, but not with Image mode?)

B. An image in Eew
==================

emacs -Q
M-x eww (enter "www.gnu.org/distros/screenshot.html" at the prompt)

A page loads with a screenshot about half way down.

B.1. Jerky scrolling:

Scroll down page with down arrow key, one press at a time.

When the cursor gets to the line above the screenshot, the image starts
scrolling smoothly upwards with each press of the arrow key, but the
cursor itself stays on the line above the image which seems wrong. If
the cursor is on a line and I press the down arrow key, I expect the
cursor to now be on whatever is below that line.

At some point (possibly when the image has risen high enough to clear
the bottom edge of the window) the cursor jumps to the right edge of the
image (at least I can see a line flashing there as if it were the
cursor). When this shift happens there is a sudden (small) jump in the
image that is not at all smooth.

One more down arrow press moves the cursor to the line below the
image. This seems correct, and the cursor continues to descend the
page. But then the page suddenly scrolls, making the image suddenly
disappear, rather than smoothly scrolling up out of sight.

If I now scroll back up the page by pressing the up arrow key, one press
at a time, at no point does the image scroll smoothly -- it just jerks
downwards in two big jumps.

B.2. No horizontal scrolling:

I maximise the frame (logo key and up arrow), then reload the page. The
image is now much larger. I now restore the page (loge key and down
arrow). I now have only the left edge of the image in the window, which
is fine. The problem is there is no horizontal scroll bar, and the image
won't scroll with the arrow keys.

C. An image in Gnus
===================

This is not from emacs -Q because I can't seem to run Gnus that way (no
gnutls?).

The image is an attachment in an email. (It's actually the same JPEG as
in A. above.)

The top left of the image is displayed. Scrolling down the email with
the down arrow key, the image scrolls smoothly until just after the
bottom edge appears at the bottom of the window, then it jumps
completely up out of view.

Scrolling back up again, when the image first appears, it does so in a
jerky motion in which the bottom maybe 1/4 of the image all appears at
once, then, on most attempts it scrolls smoothly upwards. However, a few
times it  "glitches" and the image, after scrolling fine for a bit,
suddenly jumps back to the position at which it first appeared when
scrolling up, with the bottom maybe 1/4 of the image
showing in the window.

There is no horizontal scroll bar, and the cursor keys don't seem to
scroll the image horizontally.