Factorio Forums

On the bottom left, the control hints window does state "Control + Left click" to insert a full stack, but does not state the option to insert a half stack which would be "Control + Right click" which it should.

As the name suggests, these are simply hints meant to get you started are do not include all possible relevant shortcuts. In this case, Z to insert one item is also missing.
But I will keep your feedback in mind.

Ref 119276 "Show mouse and keyboard control hints" does not work properly for vehicles.