Prevent menu button from overflowing bottom of page

[tlmii]

Fixes #2661

When clicking the ... on the bottom couple rows, it'll now go up as expected:

The 200 matches the default horizontal threshold.

Microsoft Reviewers: Open in CodeFlow

[tlmii]

There's a flicker of vertical scrollbar as the menu repositions itself, similar to what I saw when implementing #2572. The fix isn't quite as simple though, since we do want it to be able to overflow the bottom of the grid (unlike the right edge of the grid):

The flicker is less bad than not being able to access the menu, so I think this PR should still go in. I'll file an issue once this goes in to track it. I think this (unlike the original flicker) may require going a little deeper in the layers.

[JamesNK]

Is the flicker issue something for Fluent UI to fix?

[tlmii]

Is the flicker issue something for Fluent UI to fix?

My testing when doing the initial command support seemed to indicate that it was either an issue in fluent ui (the web components, not the blazor components) or a conflict between the two component layers. Filed #2837