When you're at inventory capacity, you can still drag items into their respective slots. However, trying to right-click transfer items only works up to a loadAbs of 1000, the default maxSoldierLoad. This does not affect other equipment, such as vests, uniforms and weapons. Only items transferred to uniform/vest/backpack.

Considering canAdd returns false once past this number, the player should probably be unable to pick up anything past this point.