I was messing around in creative to learn about the stuff in the pack and my hotbar got stuck. Scrolling, using my touchpad or a different mouse/keyboard, and restarting the game did not help. I can break and place what is in the slot though. Switching to survival did nothing. How can this be fixed?

I have only added Better Flight and MobNeutralizer (which didn't work anyway so I may remove it). Some keybinds have been changed but mainly unbinding a bunch I won't use at all or for a while.

Would you happen to be using the grindstone on a stick? I have similar issue now and again when using that item except restarting my game seems to resolve it.

No, I was holding a Netherite Backpack though. I grabbed another from JEI and accidentally clicked the full one (with the backpack-ception upgrade) with the empty one and my hotbar got stuck. Maybe it was a conflict with the shift drag drop fill function? Seems strange it would lock my hotbar though.

I've been getting this issue pretty consistently. Just seems to happen after a long enough amount of time playing rather than a specific action causing it. Decided to just immediately hop off and grab the game log to put here when it happened this time.
latest.log

I've got this same issue, have you managed to figure it out yet?

Nope. Nothing online either. But then again, I can't read coder speak. So nothing legible found so far. That's why I asked for help here.

I've had this issue before. Guy on reddit mentioned clicking V was the fix, he didn't know why. But yeah, to fix this issue, click V.

the cause was/is the Supplementaries quiver keybind, should be fixed in 1.0.22

It is fixed! Thank you!