Fix region splitting logic to update 1 and add 1
Fix region splitting logic to update 1 and add 1

instead of removing one and adding one, which caused segment_items
to become undraggable because an undraggable segment was covering them.

shirshendu/kine_type,shirshendu/kine_type,shirshendu/kine_type,shirshendu/kine_type