[fixed] stacked/nested modals have focus lost in Safari

[s-robertson]

If the keydown event within the currently-open, nested modal is propagated, Safari will toggle focus between the parent modal's focusable elements instead.

Fixes #801.

Acceptance Checklist:

• [x] The commit message follows the guidelines in CONTRIBUTING.md.
• [x] Documentation (README.md) and examples have been updated as needed.
• [x] If this is a code change, a spec testing the functionality has been added.
• [x] If the commit message has [changed] or [removed], there is an upgrade path above.

[marianavin]

Hey there - what's the current status of this task? It's currently blocking the development in my team. Would you be able to say if it's going to be merged soon? Thanks.