Yes, the layer is on clip level.
Hmm... the layer is set to the size of the monitor: 1920x1080 (16:9), with autosize = fit. the content has a 3/4 aspect ratio, size is 640x480. So the clip is scaled to the height of the layer. Apparently the scaling is done before the mirror effect is applied?
Even more confusing: When I move the transform effect (with neutral setting: scale = 100%) after the clip, the preview and the output match. ?!?
Maybe as a rule of thumb, it might be the best to place the transform effect after mirror (or flip, or vignette)?