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On a printer with an external camera, watching the live view while a print ran meant the layer timelapse recorded almost nothing and the finish photo went out with no image. The reporter measured 0 of 87 layer captures on one print and 0 of 105 on another, both watched throughout. A USB camera allows one V4L2 handle, so a capture during a live view fails outright. The built-in camera has had this rule since #1348 and #1271: reuse the viewer's buffered frame rather than opening a second connection. It was never extended to the external paths, and it could not have been -- the buffer it depends on was only ever populated by the built-in paths. generate_mjpeg_stream yields multipart-wrapped chunks, so the route layer could not recover the JPEG, and a guarded caller would have found an empty buffer and skipped every time. So the stream now publishes each raw frame through a new on_frame callback (parallel to on_process from #2675), and the six one-shot consumers reuse it: layer timelapse, the finish-photo moment and its background fallback, the notification snapshot, Obico polling, and the plate check. A viewer attached with nothing buffered yet skips that one attempt rather than competing -- kicking the viewer off is worse than missing a frame. on_frame exceptions are logged and swallowed, like iter_subscriber's on_unsubscribe: buffering is a side effect and must never be able to take the live stream down with it. The external stream's teardown now releases the buffered frame too, ownership-checked so a concurrent viewer of the same printer keeps its own. Two side effects on paths not touched here, both improvements: the snapshot endpoint and the finish-photo fallback chain consult get_buffered_frame and can now serve an external camera's live frame. plate_detection's docstring already claimed this behaviour while implementing it only for the built-in fallback; that drift is resolved.