An optional crop in clip mode: a Crop toggle puts a frame on the picture (free, 16:9, 9:16 or 1:1;
the whole picture stays the default). The frame is sent in fractions of the box it was drawn in,
with that box's shape; the worker fits the real picture into the box by its display aspect (sample
aspect and rotation aware), drops the part over the bars, and crops first in the filter chain. A
frame covering the whole box is no crop at all, and one off the picture is refused by name.
The crop frame is a ui primitive (CropFrame) shared with the video editor, keyboard-operable.
YouTube: the wide side panel shuts while clipping and comes back after; maximised, the clip panel
docks in the stage. Plex: the picture sits above the clip panel so the frame lies on it.
The video preview is object-fit: contain (the browser default for <video>), so a
non-16:9 source is letter/pillarboxed inside the element box. The crop marquee was
drawn in fractions of the whole box (bars included) and those fractions were then
multiplied by the source resolution, so the emitted ffmpeg crop diverged from what
the selector showed. Anchor the overlay to the computed content rect (and set
object-contain explicitly) so a marquee fraction is a fraction of the real picture;
extract the display-px<->source-px math into cropGeometry.ts with unit tests, and
clamp the even-rounded window in-bounds so a full-frame crop can't overflow.
peter