Splitting up stacks for assembly
There is!
Simply Left Click the stack you want to split to pick it up, and then right click the bank slot, inventory slot, or assembly slot, in which you want to drop one of the item in question.
When you're finished, simply left click the inventory slot you'd like to place the remainder of the stack.
Alternately, in the specific case of assembly, you can drop the entire remaining stack on the last slot of the assembly window. Assembling your item will only use one item from the stack, and you can simply use the 'quick return' button to return the stack to your inventory.