SlotStacked

SlotStacked transforms your hotbar into a vertical inventory system heavily inspired by Half-Life's weapon selection. Hold any hotbar number key for a configurable delay, and a column of three inventory slots will appear above it, displaying the items directly above that hotbar position. Use arrow keys (or scroll wheel, your choice) to highlight slots in the column, then release the key to instantly swap the highlighted item into your hotbar.

The mod is entirely client-side.

Using Cloth Config, you can choose to navigate inventory columns using either the mouse scroll wheel or keyboard keys. If you select the keyboard option, the keybinds can be customized in the Controls menu.

You can also configure the delay before the inventory slot column appears

Huge credit to William48822 for the idea.