mobile - Why is a scrollable bottom tab bar discouraged?

Why is horizontal scrolling in a bottom tab bar commonly discouraged? Both iOS and Android guidelines discourage it.

And for some reason scrolling navigation is sometimes okay at the top? (Android tabs and iOS News) And sometimes near the bottom? (iOS Camera)

I can think of these possible reasons of why it's not recommended.

• Psychologically it's counter to our expectations. We expect lower things to be more static and stable. We more easily understand fluid navigation when it's at the top area of the screen, but in the bottom we are thrown off.


• It's awkward for the human thumbs to reach down there. With the "thumb zone" that bottom right area is hard to reach.

Are there others reasons? Has anyone studied this? Thoughts?

Or is it actually okay? Is there way to do it right? Has it ever been done well?

Answer

Aside from what you already mentioned, I'd say that if you have so many items in your bottom navigation that it needs to scroll, you're doing something wrong. Material.io mentions "3-5" for a reason. Anything beyond that is too difficult to keep straight. Plus, the bottom navigation is meant to be a direct link to main pages of the app. If you have to scroll to get to them, it defeats the purpose of having them there.

Scrolling navigation works better at the top because it's almost exclusively used for subitems of a main navigation instead of for the main navigation itself. That model works better because you're searching for something more granular and would expect to scroll through 5+ items to find what you need.