This is another solution to #83. This works perfectly for me so far, and adds less complexity than modifying visibility.
Fixing hover style bug triggered every other time the picker is opened
I had this same problem with Mootools 1.4.1 and with 1.4.2
I downloaded the codes from giddie and I can confirm this fixed the problem for me!
@arian The problem is (as far as I can tell) that the oldContents container should always pushed out-of-frame unless an animation is in progress. However, when the picker is unhidden, the oldContents left is set to zero, which means it's not out-of-frame, and it causes these issues with one copy of the content obscuring the other.
The intention of this patch is to restore the expected behaviour, so that oldContents will always be out-of-frame unless an animation is occurring. For this reason, I suggest this patch should be merged ever if you're not able to reproduce the bug that it fixes: it makes the behaviour more consistent.