Open and OnLoginSuccess were each started in their own goroutine, so they
raced. Open is what marks the surface as opened on the client side, and
OnLoginSuccess does nothing until it has, so a token that arrived quickly
left the browser sitting in front of the terminal — the dismissal was
dropped rather than delayed.
The login and session-extend flows do not hit this because their two calls
live in separate functions with a blocking wait between them. Here both
are in one function, so ordering has to come from calling them in turn.
Also groups the file's helpers with the code they serve.
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