SSE is a core requirement for good operation of blueocean.
It makes sense to have a check in the background at some point that triggers an SSE callback that the front end can expect to arrive in a certain amount of time. Should this fail to arrive, the client could log a console error saying "SSE appears to not be supported". This would flush out incompatible proxies or at least eliminate SSE push events as a problem for supportability.
Needs exploration. Should SSE fail to work we can then look at the network/proxy config (but this means we don't need to guess).