A disposed PlateView stayed subscribed to its plate, so a plate that
outlived the view kept rebuilding the disposed view's layout and
raising its events; Dispose now detaches the part added, removed and
reordered handlers.
ObservableList.Reorder keeps repeated references, but the reorder
handler mapped each part to one layout, so [a, a, b] collapsed to one
shared layout for both occurrences of a. It now reuses each existing
layout once and creates one only when none is left.
PlateView now follows Plate.PartsReordered, which an applied cutting
plan raises once per changed plate: it puts its layout parts in the
plate's order (the drawn part numbers are the cutting order), marks
them dirty so new programs are redrawn, and raises its own
PartsReordered. The overlap overlay treats a reorder like any other
part change and marks its report out of date.
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