I believe this is high priority as anyone on Mac playing 1.16 has this problem:

While holding down the SHIFT key on Mac to crouch in game, scrolling with the hotbar does not work. Instead of selecting the next item in the hotbar, the scrolling will not be recognized at all. This bug does not affect earlier versions of minecraft such as 1.8 for some reason.

I believe this is because when you hold shift on mac and scroll, there is a feature where it switches to horizontal scrolling which is not recognized by minecraft in 1.16. Normally, holding SHIFT would crouch the player in game and still allow them full control over their hotbar scrolling.

I think the fix to this would be to add an option where both horizontal and vertical scrolling are recognized by minecraft as vertical scrolling in the mouse control settings in game. Or to have the game recognize any scrolling (including horizontal) as vertical, because horizontal is never used anywhere else is the game as far as I know.

This makes the game almost unplayable for people trying to play on Mac as there is no option to change this in Mac system preferences.