I was trying to add tags to my library partly to get used to the new library interface.
The window/tile/whatever-you-want-to-call-it keep flashing and jumping around. I noticed that when I do things a yellow bar pops up at the top saying it is updating the data base. This yellow bar causes the window to jump down then when the bar goes away, the window jumps up. Sometimes it takes a fraction of a second, other times around a second. Either way the result is so annoying that I don't want to bother with the Library interface anymore.