explore use cases for single input modality use cases of interest invoker

[keithamus]

#872 kicked off a discussion about how interest invokers are multi-modal (focus/hover) and sometimes that's not applicable.

We should try to discover use cases where that isn't applicable. Two mentioned:

• Menus where you'd want sub-menus to open on hover, but not focus
• Carousels where you'd want the list of pages to rotate with focus, but not with hover ([invokers] invoke on focus but not hover #1030).

We should prototype these out and see what primitives are missing and how we can address these.

[lukewarlow]

Something I've thought of with the menu case is how often do users actually like the open on hover? It might be that they should just be an invoker? The thing that focus isn't strong enough for a menu open might just apply to mouse users and hover too?

[keithamus]

I think it is reasonable to make menus open on hover. Pointer devices are quite intentional for those who regularly use them, and having to click to affirm each action is more work than necessary for ephemeral actions like showing sub menus.

[lukewarlow]

See #838 for a potential use case