feat(menu): Support fallback positioning on menu trigger.

[trshafer]

This ensures that the menu will do the best attempt at showing instead of rendering off screen.

[jelbourn]

I believe this was already addressed in #1761. @kara can you confirm?

(FYI, be sure to let us know if you plan on doing work to something under active development like menu or select so we can avoid conflicts)

[trshafer]

The other PR looks good. Thanks for merging it in.

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