In Survival mode the player is unable to drop items or stacks from inventory by dragging them outside the GUI and clicking. This is only the case for the player's inventory, as discarding works as usual in all other interfaces, such as chest, workbench, anvil, etc.

To be a bit more clear, if I want to destroy a stack of cobblestone by throwing it into lava, I drag it outside the GUI and click to drop it. In this snapshot, that no longer works. When performing the final click to drop the stack, it remains attached to the pointer. To discard the stack now, the player needs to have it on the pointer when exiting the GUI. (Grab a stack, press 'E' twice, grab the next stack...)