Open pages recover when their hub restarts

When a page's code is still being compiled, the portal shows a short "compiling" placeholder and then restarts the page's server side so it picks up the finished build. Anyone who arrived after that restart got the real page. Anyone who was already looking at the placeholder did not: their page was never told the restart had happened, so it kept showing the placeholder — with no error and no spinner — until they reloaded the browser.

That was easiest to hit right after an upgrade, which recompiles every page's code at once and so opens the window on every open page in the portal at the same time.

A restart now tells the pages it was serving, and they re-connect on their own as soon as it finishes. The page you were already looking at fills in by itself.