Display engine bugs

[Juri Linkov]

I found some bugs related to text displaying:

1. When a point is located on the second line from the bottom of the
window, then scrolling down one line (e.g. by C-u 1 M-x scroll-down)
moves point to the beginning of the current line.  This is a bug
because the current line becomes the last line of the window and it
remains visible, but its point moves to the line beginning.  I have
also a related feature request: to preserve column position if a line
becomes invisible after scrolling, i.e. to move point to the same
column of the previous visible line instead of jumping to the line
beginning as it does now.

2. Scrolling the buffer screenfully with text in a proportional font
sometimes moves a point into center of window instead of moving it
into first or last window line.  Sometimes scrolling down towards the
beginning of the buffer stops on the last line of the first screen
and don't move further and don't signal an error.

3. When window is split vertically (e.g. by C-x 2), the cursor
(hollow box) don't appear immediately in the non-selected window.
It appears here only after pressing some key or after some timeout.

4. "Black hole"

Copy into scratch:

(progn
  (setq tab-width 8 truncate-lines t)
  (mapc (lambda (l) (insert-char (car l) (cadr l)))
        '((?\n 1) (?\t 3) (?\040 5) (?a 75) (?\040 1) (?\n 1) (?\t 16))))

and press the following keys:

C-j C-p C-b C-b

now start to press <backspace> may times until you see a black hole.

blackhole.png
Description: blackhole

5. Next bug can be reproduced by evaluating the following line:

(progn (setq tab-width 8 truncate-lines t) (split-window-horizontally))

Now set left window width to 82.

Insert the following line into buffer:

                           
aaaaaaaaaaaaaaaaaa(xxxxxxxxxxxxxxxxxxxxxxxxxxx);(yyyyyyyyyyyyyyyyyyyyyyyy)

and press the following keys:

C-e C-SPC M-C-b M-C-b C-w

On the next screenshot the first line shows how it should look,
the second line show how it actually looks.

glitch.png
Description: glitch

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