bug#6264: 23; scrolling up erratic when line fractions visible at bottom of window

[David Reitter]

Moving down in a buffer does not work well at all when the last line in the 
window is not fully visible.

The problem occurs in a wide variety of conditions, but here is a test case 
that works for me:

Emacs -Q  [23 branch build, NS port]
(setq scroll-conservatively 100)
(setq line-spacing 0.6)
(modify-frame-parameters nil '((height . 20)))

Insert some 40 lines of text into the buffer.

Now set point somewhere near the beginning of the buffer, and press <down> 
repeatedly to scroll the buffer.

You will see at some point that the text is scrolled up by more than one line 
so the cursor moves vertically up and down. Screenshots down-0 and down-1 
illustrate this.

What should happen is that the window scroll so that the baseline of the last 
text line stays the same, analogous to what happening when we're at the top end 
of the window, pressing <up>.

NOTE: the above works with the default font (Monaco) on my system; on other 
systems or with other ports, slightly different line-spacing would apply.


Another, perhaps related problem:
Bottom line (before scrolling starts): here we have a buffer with lines that 
have different heights (AUCTeX).  
We've got scroll-margin at 0, scroll-conservatively at 10000,  
scroll-up-aggressively nil.  The attached screenshot (bottom-half-line) 
illustrates the problem: the last line is half-visible (that's OK), but the 
window scrolls too late.

The problem can be seen in the above Emacs -Q session with (setq line-spacing 
1.2).