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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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Cookies's use case is to store persistent data and send it to the server
in subsequent requests, such as to remember logged-in sessions. WebAO is
using them to store site settings like ad-hoc hash tables that require
parsing and serialization.
As a nasty side-effect of how cookies work, clients send all their
settings every time they connect to the server. Server has absolutely no
use for them, but each client sends them anyway, which is an
uncalled-for privacy leak.
Remove this mechanism entirely, switch to localStorage which serves
exactly the purpose of per-origin store with data that never leaves the
browser.
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