SPIKE: Browser focus doesn't shift to annotation when using VoiceOver + Safari for Mac on first annotation

### Steps to reproduce
1. In Safari for Mac, Go to any web page or document in Safari - I've been using [this one](https://webaim.org/techniques/screenreader/)
2. Activate the Hypothesis Bookmarklet
3. Ensure VoiceOver is activated
4. Create an annotation using the keyboard - use cursor navigation to select your text and then use the "a" keyboard shortcut

### Expected behaviour
A new annotation draft is created. The browser's focus shifts to the editable text area of the annotation body. This is announced by VoiceOver

### Actual behaviour
A new annotation draft is created, but the browser's focus remains on the text selection within the body of the page. A blind user would think nothing happened.

If you manually click the "Cancel" button on the draft annotation, the next time you go to annotate it will work.

### Browser/system information
timestamp: Fri Apr 17 2020 14:47:21 GMT-0400 (EDT)
url: https://webaim.org/techniques/screenreader/
fingerprint: N/A
account: katelyn (acct:klemay@hypothes.is)
userAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Safari/605.1.15
version: 1.391.0

### Additional details
I made a screencast and shared in Slack. View it here: https://hypothes-is.slack.com/archives/C8TPC8XMK/p1587138926008800

From Rob:
> I wonder if there might be a security / anti-abuse feature in Safari that prevents an iframe getting focus until a user clicks in it.
> Either way, I’d consider this a bug, but depending on what Safari is doing I’m not sure if the sidebar will be able to “steal” focus without user interaction.

...if there is no way to "steal" focus, we should find some way to help users make that initial interaction, even if that is documentation.