Timeline panel stays on Loading… in a background tab (hidden page never does the first fetch) #259

On B (main 7922771) the Timeline panel stays on Loading… for more than 20 s while A's renders. timeline/window answers in ~2 ms on both instances and B's real payload renders fine, so it is not the endpoint, a writer lock or the trace size.

The only path that leaves the panel in loading without a request: the poller does nothing while the page is hidden (document.visibilityState), including the first fetch. A panel opened in a background tab (or a window that is behind another) says Loading… until it is looked at, and then waits for the next timer.

Fix: a hidden page still loads once; it stops polling afterwards; becoming visible again refreshes at once.