A mixed filament slot is virtual: ToolOrdering::resolve_mixed_filaments()
replaces it with its physical components before any G-code is emitted, so
the toolchanges the prism has to absorb are between those components.
ensure_belt_purge_tower() counted the slot as a filament of its own,
provisioning one island per mixed slot that no swap can ever reach -- the
"extra purge tower" on MCTEST5, where filament 5 is a 50/50 blend of 2
and 4 and the G-code reports 0.00 g of it used.
Expand the assigned set with the same expand_mixed_filaments() the
backend uses, so the GUI sizes the prism against the filament set the
slicer actually produces. No-op when nothing is mixed. Test covers the
MCTEST5 shape, a mixed slot whose components are otherwise unused, and
the no-mixing case.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SsuY8Laiyh7q2zPVVKV3HZ
