Fix #3: issue with Webkit jumping on position:absolute

I finally nailed down #3: the issue was happening when the sticky sidebar was set to `position:absolute`.

The problem was that the CSS was set in 2 independent `.css()`calls : one for `position` and `top`, then one for `width` and `left`, which caused Webkit to redraw in-between.

Setting all 4 properties in a single `.css()` call fixes the issue.