Fill Routing: Label Authority

Status: placeholder — content pending. Fill freely in English or 中文.

Scope

The correctness rule that fixed the "fill C same colour → A changes" bug (a7700df, 2026-08-27):

flowchart TD
    C["click on face F"] --> L["look up segmentLabels[F]"]
    L --> T["fill EXACTLY that region (by label)"]
    T --> O["paint it with the selected colour<br/>(colour is output, never identity)"]
    X["hover snapshot / stale cache"] -. retired · never an input .-> L

Topics to cover: labels are truth; colour is output — fill targets strictly the region the clicked face belongs to, so two regions can share one colour; the retired iter29 hover preference and v5 stale cache (lastValidHoveredSegmentRef); regression guarantees in commands::paint::fill_tests (same-colour double-region fill must not modify the first region; Shift+click flood stops at label boundaries); where the rule generalizes (fuse/grow/lasso must propagate labels, never overwrite face_colors — open backlog item, see ../04 §8).

Code pointers

• src-tauri/src/paint/fill.rs — core fill + tests
• src-tauri/src/commands/paint.rs — fill_paint / fill_segment_paint
• src/hooks/usePaintTool.ts — frontend target routing

TODO

• Decision record: why clicked-face label beats hover/memory heuristics
• Fuse/grow repaint ruling & the 3MF colour-strategy decision it depends on