Within fx() (which is used by animate()), the css 'overflow' property loses it's original setting resulting in a "pop" back to a non-clipped state when one is trying to shrink the height of a div while using an overflow of hidden.

easy to fix:

change line 4954-5 to include a save of the original state:

y.origOverflow = y.overflow;    // NEW LINE
y.overflow = "hidden";

then revert to changed state by changing line 5067 from:

y.overflow = "";    // OLD LINE

to:

y.overflow = y.origOverflow;    // MODIFIED LINE