I regularly find myself in a situation where I want to sell / ship a large number of items, but by no means all of it. I'm aware of right-click to move the whole amount. Which is fine if I want to send all, or almost all off it. But a 50/50 split (or 60/40, or whatever) is pretty time-consuming if you have many hundreds or even thousands of items.
(I'm a little afraid that someone has implemented a solution a long time ago and I just haven't found it yet. If that is so, please tell me.)
I imagine a modfier (like CTRL-Click or Shift-Click) to move half of the amount you currently have on the providing side:
units in storage | units to ship / sell
at start: 1000 | 0
1st click to sell 500 | 500
2nd click to sell 250 | 750
3rd click to sell 125 | 875
1st click to un-sell 563 | 437 # moves 1/2 of 875, give or take rounding errors
