Exact

Preview and export produce the same pixels, within codec noise.

Always exact: geometry (crop rect, rotation, output frame size, letterbox bars, overlay position, scale and anchor), timing (trim points, concatenation boundaries, total duration), presence and absence (mute, track selection, which overlays are drawn), and text layout (line breaks, metrics, measured extent). If any of those differ, that is a bug in filmstrip.

This is preview against export on one platform. Android and Apple can still differ from each other, which is reported as FilmstripError.PlatformEffectAsymmetry.