serializeCanvasShapes recomputed each region's owner purely from geometry,
ignoring the persisted question_id: a region with no enclosing part (or any
region when a template had no parts) was silently dropped, and one overflowing
its part by a pixel re-parented to parts[0].
Resolve the owner in order: geometric containment (a drag into a part re-attaches)
-> persisted question_id if it still points at a saved question (survives drift /
no-container) -> nearest part on the page -> first part -> first question of any
kind. exam_response_areas.question_id is NOT NULL, so a region is never dropped
while any question exists.
Tests: never drops a region with no enclosing part; persisted question wins over
geometric nearest. Full model.test.ts suite (6) passes; tsc clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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