[popover2] feat(ContextMenu2): support popoverProps.rootBoundary

[GalvinGao]

Fixes #5411

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• To enable overriding the default rootBoundary value in ContextMenu2 component.

Reviewers should focus on:

• Default value done right? (or not?)

Screenshot

N/A: I'm unsure exactly how to test such change. I've tried to just add a Example component in the docs by following what other documents appear to do, but I found that the problem mentioned in #5411 is not reproducible within the docs-app for unknown reasons. Therefore I'm unsure if adding the rootBoundary props to the ContextMenu2 will actually open the door for developers to mitigate this issue. Any help would be appreciated.

[palantirtech]

Thanks for your interest in palantir/blueprint, @GalvinGao! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

@GalvinGao I edited your change to have a more minimal code diff.

I think we should test this change in the docs app or a unit test before merging. You could add a ContextMenu2 to the Drawer example in the docs-app for testing: https://github.com/palantir/blueprint/blob/develop/packages/docs-app/src/examples/core-examples/drawerExample.tsx

You may need to set boundary instead of rootBoundary to fix the problem. See the popper.js docs: https://popper.js.org/docs/v2/utils/detect-overflow/#boundary

[adidahiya]

@GalvinGao are you still interested in this PR? Can you add a unit test?

[GalvinGao]

@GalvinGao are you still interested in this PR? Can you add a unit test?

Just got time. Gonna add it today or tomorrow.

[GalvinGao]

Added docs & unit tests. Ready for review ;)

[adidahiya]

looks good