erratic scrolling on incomplete lines

[Sébastien Kirche]

Hi,

since  a moment I  have sometimes  an erratic  scrolling when  pressing PgUp
(that is bounded  to scroll-down), and i think i managed  to figured it out.
(I am  currently on OSX/Aqua  but IIRC i  have also seen  that in X11  in my
GNU/Linux system)

Problem seems to be  that the frame is dimensioned such as  the last line of
the buffer (except the minibuffer)  is truncated vertically. I though it was
not possible  as Emacs only sizes  by character dimensions  increment (i use
the 6x12 font).

Then if, when scrolling the buffer, the cursor is positioning on a line that
would be the  last but incomplete if displaying the buffer  from the top (at
this moment my cursor is centered vertically) :
1) the next scroll-down step displays the buffer from the top with the cursor
  on the last line
2) another scroll-down redisplays the  buffer with the cursor centered (point
  doesn't move). Any further scroll-down will cycle to the 1)

I hope that my description is clear enough...

Regards.

-- 
Sébastien Kirche