- Scale the document to the selected output resolution. It was rendered
at document size regardless of the export dimensions, so picking a
different resolution didn't scale the stage.
- Run the audio+video mux on a background thread instead of the UI
thread, keeping the app responsive (showing "Finalizing") during the
re-mux pass.
- Send desktop notifications fire-and-forget. notify_rust's show() is a
synchronous D-Bus call that blocked the UI for the full service
activation timeout (~25s) when no notification daemon is running.
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