Re: How to scroll images in Emacs?

[Eli Zaretskii <eliz@gnu.org>]

> From: nljlistbox2@gmail.com (N. Jackson)
> Date: Thu, 20 Feb 2014 00:20:08 -0400
> 
> 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.

Vertical scroll bars work for me, if I click on the arrows at the
edges of the bar.  The thumb size does not reflect the displayed
portion of the image, but that's at best a missing feature (Emacs sets
the size of the thumb by counting buffer text, not pixels).  If you
see something else, it could be a GTK problem.

> Horizontal scroll bar is missing entirely from window.

Emacs doesn't support horizontal scroll bars, never did.

> 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.

Again, doesn't happen to me.  Suggest to report a bug with the
relevant data.

> 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.

That's how Emacs is supposed to behave, so no bug here.

> 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.

If you want that, turn off line-move-visual (but then you'd lose
smooth scrolling of images, I think).  By default, line scrolling
scrolls by pixels in this situation, which leaves the cursor on the
same line, as long as the cursor is even partially visible.

> 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.

That's because Emacs must leave point visible on the screen at all
times.  Again, this is the intended behavior.

> 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.

Again, expected behavior.

> 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.

Not here, at least not with images I tried.

> 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.

What are the arrow keys bound to?  If they are not bound to horizontal
scroll commands, like image-mode does, you won't see any horizontal
scrolling.

> 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.

Expected.

> 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.

Doesn't happen here.

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

See above.