Note: This report IS a duplicate of MC-68743, which was falsely closed.
As my comments were ignored I saw no other way to help resolving this issue, than creating a clone of it.

When trying to evenly split a stack with the middle mouse button (mouse wheel) it is trying to place a whole stack into the slots instead of a splitted stack. It worked in version 1.7.10 and lower. (Since the stuff was added)
Also, the "remaining items" number at the orginal stack is reduced by 64 per slot, resulting in red negative numbers.

This occurs with every stacksize >= 2.

A video showing this bug:
https://www.youtube.com/watch?v=6KVN0GwGDCI