Left or right-clicking while holding an item stack has different results in different parts of different inventories.
✔ = makes sense
✘ = needs change
Most Inventories and Containers:
✔ Right and left-click both drop items only when outside the GUI area.
✘ Right-click drops an item anywhere unless you're hovering over a slot.
✔ Left-click drops the items anywhere except for when hovering over a slot or tab.
✘ Right and left-click both fail to drop items in the area where tabs can appear, even when not hovering over a tab.