Reading a diagnostic
Web warnings other than UNSAFE_URL_BLOCKED are not deduplicated by code (each is described below).
Development builds carry the full text: [shadow CODE] message. Warnings go to console.warn. Errors are thrown: ShadowError (core) with a code property, SsrError (server renderer) with a code property, a TypeError/RangeError whose message starts with [shadow CODE], or a CompileError (compiler) whose diagnostic.code is the code.
Production builds keep only the code, so message strings never reach your bundle. formatMessage(code) returns SHADOW_<CODE>, a ShadowError message is SHADOW_<CODE>, and warn compiles to nothing. Search your logs for SHADOW_ followed by the code from this page.
development: [shadow WRITE_IN_MEMO] A signal was written inside a useMemo() computation. ...
production: SHADOW_WRITE_IN_MEMO
Once per code. Core's warn reports each code once per module instance. setWarningListener(listener) redirects core warnings to your function (devtools, error reporters); it returns the previous listener.
Web warnings other than UNSAFE_URL_BLOCKED are not deduplicated by code (each is described below). The router's warning is not deduplicated either.