The gateway.ping heartbeat failed the transport when the newest inbound
frame was older than the 45 s deadline. A hidden or minimized Chromium
window runs the 15 s heartbeat interval once a minute (intensive wake-up
throttling), so at every tick the previous ping's pong is ~60 s old even
though the backend answered it within milliseconds. Every idle hidden
window closed a healthy socket, redialed, and the renderer recorded
hermes.desktop.friction kind=backend_disconnect detail=network for it.
Judge silence only when the newest ping we sent is still unanswered
(lastPingAt > lastLivenessAt). A dead transport still fails one tick
after its first unanswered ping, throttled or not; an answered one is
never dropped for timer spacing. This also covers the suspended-process
shape (OS sleep, WebView freezer, SIGSTOP): the pre-suspend ping was
answered, so the first tick after resume pings instead of failing.
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