isRenderingDiff was only ever cleared by markDiffRendered() on .didRender — if YiTong's DiffView reports .didFail instead, the loading spinner would stay up forever with no indication anything went wrong. - Add markDiffFailed(_:), clearing isRenderingDiff and storing the DiffError in a new renderError property - Route DiffView's .didFail event to markDiffFailed in DiffWindowContentView, showing a small error message in place of the spinner - updateDiffDocument clears any prior renderError once a new document starts rendering, so switching files (or refreshing) naturally "retries" without a stale error message lingering - Add tests for both paths