The Position motion path drew a key's spatial handles only where its
tangent wasn't zero, and auto-Bezier keys have none at the path's ends,
so a two-key path (the usual start) had no handles to pull it into a
curve. Auto-Bezier keys now show a handle on such a side too, pointing
where the path leaves the key, a third of the way to the next key;
dragging it makes the key Bezier with that tangent. Linear keys still
show none.
Keyframe Interpolation > Spatial Bezier copied those zero tangents, so
the keys stayed straight with no handles (and read back as Linear); it
now gives them the same handles. The path keeps its shape and timing.
The handles come from the new keyframe::spatial_handles.
Tests: spatial_bezier_gives_a_straight_path_handles (engine),
a_straight_motion_path_has_handles_to_curve_it (ui_viewer).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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