He Steffen,
I am glatt that you opened this forum as the comments were exploding as you mentioned on the site.
Anyway: I have problems understanding the transparent div part that appears above the "block" that contains the border of your first block on the your full-feature test page:
http://www.ruzee.com/files/shadedborder/test.html

It's a bit hard to explain, so will do my best, don't run away ;).
With firebug I noticed that the first (black width) border block contains a negative top of -16 (related to the border) such that a div starts already 16px above the visible block. Firebug shows that nicely.
The problem I am having with this transparent part is that it doesn't let me click my button if it lies just above the border block in/below the transparent part. Neither does the hovering works on the button.
How can I solve this (without making my button part absolute)?
Below you find the Xpath in your test page to the block that I mean and the div part of that block:

-- Ed

/html/body/div[2]/div/div[6]