I don't know how my post could have been missunderstood this bad...
I have no issues double right clicking items, it's always on "give all to retainer" first.
The problem is not knowing which stacks he already has.



This is what I meant: getting a clue on stacks that I carry, which also are on the retainer, so I can without wasting space or time give him my stuff on top of his.

I want to see and know instantly what I can dump on my retainers to make space, as fast as possible, with the highlight match maker shown in my suggestion here.