WIP - Fetch annotations for each frame separately

[robertknight]

Explore a client-side solution to sending annotations to correct frame
if annotation URL does not exactly match frame URL (see [1]). In the
client-side solution, a separate search request is made for each frame
and the returned annotations are then associated with that frame.

Some issues to resolve:

• How can we test this locally with the dev server?
• What if searches for different frames return the same annotation?
  • Allow annotations to be associated with multiple frames by making the $frameId property an array?
  • Preferentially associate the annotation with the main frame?
• How should the WebSocket be adapted?
  • A separate WebSocket connection per frame?
• What do about frame IDs for non-sidebar applications (notebook, stream, single annotation page) where annotations are not associated with frames?
  • Treat the annotation these views like the sidebar when there is only one main frame?

[1] #4191

[robertknight]

How should the WebSocket be adapted? A separate WebSocket instance per frame?

This is not ideal, for a few reasons:

• Some WebSocket messages pertain to annotations, but others pertain to objects which are not associated with frames, such as the user's groups
• WebSocket connections have a per-connection cost on the backend
• On the client side there will be added complexity for managing multiple WS connections

On the client side the main problem with WS notifications currently is that we don't know which frame(s) a new, but not updated or deleted, annotation should be assigned to. If the backend could provide information about which URIs in the WS configuration the annotation matches, that would solve the problem.

In the backend it looks like it should be straightforward to determine which URL in the WebSocket configuration matched a particular annotation and include that information in the response.

[robertknight]

Related Slack discussion.

[robertknight]

How should the WebSocket be adapted? A separate WebSocket instance per frame?

A possible incremental step is for the WebSocket to initially only receive updates for the main frame, or whichever one connected first and is still connected.

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