bug#29374: patch for lisp/pixel-scroll

[Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>]

>> Date: Tue, 21 Nov 2017 13:35:07 +0900 (JST)
>> From: Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>
>> Cc: tkk@misasa.okayama-u.ac.jp
>>
>> I'm sending a patch for lisp/pixel-scroll.el.
>>
>> This patch fixes movement of cursor to stay away from the top of the
>> line and the end of the line on a screen.
>
> Thanks, but please tell more about the problem, and perhaps show a
> recipe to reproduce it.

First of all, expression of my previous message was not correct.
Let me rephrase as below.

   This patch fixes movement of cursor to stay away from the top and
   the bottom of a screen.

Before fix, sudden scope-jump happened on certain cases.  Let's
consider a case with cursor is located at the bottom and about to
scroll down (move scope up).

There is a condition where
 bob is shown on the top of screen
 and window-vscroll is non-zero.

On this condition, (scroll-up 1) would be called without
(vertical-motion -1).  As a consequence screen jumps.

After the fix, cursor will be moved by vertical-motion before calling
scroll-down or pixel-scroll-pixel-down.