A Safari/Firefox/Helium user who merely had Chrome installed watched Chrome open on every desktop update (community report). start_ui now checks the system default browser (LaunchServices https handler on macOS, xdg-settings on Linux) and skips the shim window unless the default is Chromium-family; notify_fallback and the durable result file still carry the outcome. Detection is best-effort: any failure keeps the old behavior.
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