phosphor-shell-osd

The Phosphor.OSD framework owns the transient overlay a shell flashes when you change the volume, the brightness, or toggle the mic. OSDHost shows one at a time, holds it for a timeout, fades it out, and routes it to the right screen, debouncing repeated triggers so spinning a volume key keeps one surface alive and refreshed instead of stacking a dozen. The framework owns presentation and timing only: the content comes from delegates (four ship built in, and plugins can contribute more) and the triggers come from the host, whether that is a hotkey handler, an IPC call, or a service signal.