As a concrete example (steps to reproduce):
- Clear any existing mapping for the input "Middle Mouse Button"
- Clear any existing mapping for the action "Stack split"
- Map the "Stack split" action to the "Middle Mouse Button" input
When viewing the contents of a chest, middle-clicking on a stack in your inventory will transfer half the stack to the chest.
I searched for this bug and couldn't find it, though I found this resolved bug which seems similar: