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CH is an application-level keepalive packet that clients periodically
send for two reasons:
1. It tells the server they're still connected, preventing timeouts.
2. By measuring latency between sending CH and receiving CHECK, a client
can display ping.
Keepalive is redundant because WebSocket can handle that via PING frames on a
transport layer. WebAO also completely ignores CHECK and sends CH every
five seconds, which is superfluous (AO2 Client sends it once every 45
seconds, in comparison).
Sending CH via `setInterval` was also problematic: browsers seem to
throttle it when the tab becomes inactive, preventing periodic pings and
leading to the server disconnecting inactive browser clients.
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