Scroll bar size marker on w32

[Lennart Borgman]

I just noticed that the scroll bar size marker (the thing that moves on 
the scroll bar) changes it size as you scroll a buffer in Emacs on w32. 
I have not seen this in any other program on w32 and it seems to me to 
be against the expected behaviour.

Is this something that is w32 specific?

To notice the the effect make the window a little bit smaller than the 
buffer and move to the top and bottom.