Align positioning behavior of rich-text mention listbox, combobox, select, and menu button

[m-akinc]

Pull Request

🤨 Rationale

The mention listbox only opens down, this change allows the mention listbox to open based on available viewport space. Fixes #2246

👩‍💻 Implementation

Of the controls that implement anchored region, the select and combobox are most aligned to the rich text mention listbox as they are all Listbox implementations with sized listboxes

• menu button uses a simple pattern for positioning that is unaware of popover sizing
• tooltip has more detailed positioning that is unaware of popover sizing

This PR updates the richtext mention listbox to align with the select and combobox sizing behavior but does not bring forward the configurable positioning behavior.

• rich text mention listbox will use a fixed dynamic positioning because it does not have a user configurable positioning like select and combobox.

PR also includes:

• minor update to align on the shared DropdownPosition enum consistently
• minimum dropdown height of 1.5 options (for dropdowns that are confined to the window height)

🧪 Testing

• Manual testing in Storybook
• New visual tests for default positioning behavior of combobox, select, and mention listbox

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[jattasNI]

I've been holding off reviewing this since it looks like there are active discussion threads which I don't have much of an opinion on. Please let me know if you're expecting my input on any of them, otherwise I'll take a look once they're resolved and my vote's reset.

[m-akinc]

Removed stale comment.

[m-akinc]

We weren't providing a way to demo the default (unset) dropDownPosition behavior.

[m-akinc]

Like with the combobox stories, we weren't providing a way to demo the default (unset) dropDownPosition behavior.

[m-akinc]

Simpler and harmless to add padding to both top and bottom of anchored region without worrying about its positioning.

[m-akinc]

@jattasNI @rajsite Obviously not urgent, since this hasn't been active for several months, but I don't think there's anything blocking this. Review when convenient.

[m-akinc]

@jattasNI @rajsite Another ping. Still not urgent, but if there's any development on these dropdown controls in the meantime, it could get difficult to merge.