When adding storage capacity, the game will auto-fill that location with stocks immediately. Since stock will not leave the location on exclusion, would it be possible to get a toggle to open/close storage locations and have them on “closed” when delivered. This so you can set storage groups before opening the building.
Maybe also add a “housekeeping” task to move out all stock that is not allowed to be in a storehouse, so you can organize your city to your wishes. Now you have to wait until the algorithm by accident is freeing up the room you wait for.
