This mod adds hotbar swapping with mouse buttons to 1.12.2 & 1.7.10, a feature that was only added in 1.16.
Hotbar swapping is when you hover over an item in your inventory and press a hotbar key (1–9) to send it to that hotbar slot without dragging it with your mouse. However, earlier versions of Minecraft didn't let you do this with mouse buttons. This mod fixes that.