Retaining a line's length added a P2PDistance constraint, whose gradient
divides by the current distance. For a zero-length line the solver got a NaN
Jacobian and its SVD fallback never returned, freezing the editor (e.g.
ECONSTRAINT on a line drawn by clicking one point twice). Hold coincident
endpoints together with per-axis differences instead.
Control-point splines read without knots are given a degree their control
points can carry (at most n - 1) and a clamped knot vector when a drawing
loads. Generating knots for fewer points than the degree underflowed during
DWG write: a panic in debug builds, an unbounded allocation in release, and
reachable from autosave. A lone control point with no knots or fit points is
purged as corrupt.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
