Since Apple released Yosemite, it’s been kind of puzzling–most people have had no special trouble with PSE 12 since the update, but some people have been having a heck of a time with any of the tools in the Enhance section of the PSE toolbox causing PSE to freeze completely.
It appears that the problem is something to do with trackpads only, so for now, if you switch back to using a mouse everything should work again.
