Add support for inter-frame communication on the same tab

[twschiller]

Context

• The PixieBrix sidebar in the context of the top-level frame
• The PixieBrix content script runs in the context of the frame its in
• The PixieBrix "broadcast" target exclude the sender tab
• Documentation: Editing pages with iframes
• The PixieBrix Page Editor only operates on the top-level tab. This issue is for enabling some enterprise use cases on Salesforce and Service Now which use frames

Options

• Allow PixieBrix to target sub-frame by URL
• Allow PixieBrix to target sub-frame frame selector (and PixieBrix figures out the frame id, URL?)
• Allow PixieBrix to target top-level frame
• Allow PixieBrix to broadcast to all frames on the page

Assumptions

• The creator must give PixieBrix permissions to run on both the tab and the sub-frame (so the content script is loaded on the sub-frame)

Discussion

• The easiest path for now is to support a "tab" target that runs on the top-level frame. That would allow sub-frames to publish context to the top-level frame using "Set Page State"
• Long term, we'll want enable more seamless brick execution in subframes, which will be the most common direction

[fregante]

Very related:

• How can a content script target a frame? webext-messenger#41 (comment)

I left a message there, but #3201 looks good as is too.

[github-actions]

This issue will be closed in 7 days unless the stale label is removed, or a comment is added to the issue.

[twschiller]

Closing, because we've implemented support for:

• Allow PixieBrix to target top-level frame
• Allow PixieBrix to broadcast to all frames on the page