I'm having this problem as well, with unmodded 1.6 snapshot, and I already tried updating my lwjgl. I'm on a Mac, and holding Control while clicking on a stack of any item does not make it split evenly or allow me to place single items in inventory/crafting slots. Makes it very difficult to craft.
Hugely seconding this issue. In a new world I made to try out with the current snapshot, I ended up accumulating a ton of stuff, and wound up with a horse, and a donkey and three mules to carry everything. After flying around to find a place I wanted to take all my stuff and settle down to build, I set off on foot with my animals, and immediately ran into trouble with the leads breaking. I ended up having to stop halfway there to leave three of them tied to a post while I continued on with the other two, and even then I still had leads breaking. It does seem to happen when they stop to graze while I'm still moving, and if I pass through an area smaller than their perceived hitbox size. A trip that I was hoping would take me ten, maybe fifteen minutes, ended up taking a little over a half hour, because I kept having to walk backwards or stop and look back to do a headcount. Going through a forested area was agony. I've seen it stated in other bug reports that the leads don't have infinite strength, but this is ridiculous.