I've updated the archive in the first post with a curtain fix by 
leesester, sent to me by 
fanseelamb (Thanks guys!). Here's what she wrote about it:
It was a bit of a strange bug - when the curtains were placed over a table with booth seating, sims couldn't sit in the seats next to the windows. [...]  Curtains already on the lot must be deleted and re-bought for the fix to take effect.  It also fixes a bug where the curtains couldn't be placed over toilets, and perhaps some other objects.
I'll also add it to this post if you don't want to re-download the rar. Just replace shiftcurtain.package with this file.