During development and testing of Bug 1772213 it was discovered that there were some pre-existing problems with websocket functionality when using an Http2 proxy.

Some improvements were made to the added test cases created in Bug 1772213, but an edge case still exists: When serving multiple h2 websocket client connections over a proxy, our tests will fail. Note that it doesn't seem to matter what kind of proxy we use and it only happens with h2 ws.

Existing tests reside in netwerk/test/unit/test_websocket_server_multiclient.js