The slide effect - .hide('slide') / .show('slide') - does not work under these conditions:
1) BODY or container-element has: dir="rtl"
2) Effect-element has: position: absolute;
3) Effect-element has: right: 5px; (any value)
In this circumstance: On hide() the element's size fluctuates slightly and then it disappears; The reverse happens on show().
If EITHER the 'rtl' or 'right' values are removed, everything works fine.
This affects ALL versions of jQuery and UI, including jQ 1.4.4 and UI 1.8.6. I tested only in IE7/8 and Chrome - it is identical in both browsers. I assume other browsers will react the same way.
Here is a demo page that shows the issue...
A 'left' position is assigned when this effect starts. At the same time, the 'right' position should be removed (after saving it). When the animation completes, reset the right position to its previous value.
The demo page simulates this fix. It can be enabled & disabled using the button at the top of the page.
As far as I know, this CSS modification could be applied without problem under ALL conditions, for all animations, since the effects module always calculates everything *from the left*.