Each ribbon panel was rendered five ways every frame so CollapsePanels could
roll back to any density at layout time: full, compact, button, tight and a
full-size flyout that duplicated the full rendering.
The layout pass ran four full measure-layouts per panel even in forced modes,
and the degradation decision was an untested u8 inline in Ribbon::view.
Now the choice is a pure, unit-tested decide_levels() whose levels are a typed
Level enum (Full/Compact/Collapsed/Tight). Panel carries its four renderings
as elements: [Element; 4] indexed by Level, with a slot(i, level) helper for
the tree stride. Forced modes (Full/Compact/Collapsed) measure nothing at all
and derive the row height from the placed panel. The open flyout draws the
panel's own `full` rendering inside a self-drawn box (identical weakest-bg +
1px neutral-border to the container it replaced), so no duplicate
render_group is built per panel per frame. Level, Panel, Widths, CollapsePanels
and decide_levels are pub(crate); CollapseMode stays pub (re-exported).
Tests (collapse.rs tests module, added RED first):
- forced_modes_pin_every_panel
- auto_huge_width_keeps_everything_full
- auto_degrades_from_the_right_one_panel_at_a_time
- auto_stops_degrading_once_the_row_fits
- auto_escalates_a_single_panel_past_collapsed_to_tight
- auto_cascade_drops_every_collapsed_panel_to_tight
- auto_runs_the_compact_phase_across_the_row_first
- no_panels_stays_empty
- single_panel_walks_the_whole_ladder
- zero_widths_never_overflow
- endless_row_degrades_everything_to_tight
- unbounded_row_keeps_everything_full
- nan_budget_degrades_both_phases_but_no_tight_cascade
- exact_fit_at_full_stays_full
- matches_the_pre_refactor_algorithm: byte-equivalence sweep against the
verbatim pre-refactor u8 algorithm (280 width/budget combos incl. the
degenerate 0/0/inf/NaN inputs), proving the decision logic is unchanged.
Measured with the committed #[ignore]d benchmark
(`cargo test --lib --release -- --ignored --nocapture bench_ribbon_view_construction`)
run as one instrument across pre-change / this change: Ribbon::view()
construction ~676 -> ~425 us/frame (~37% of view construction saved); forced
modes run 1 layout call per panel per frame instead of 5.
Verification: cargo build clean; cargo test --lib = 382 passed (367 baseline
+ 15 new ribbon tests) / 2 known pre-existing failures unchanged
(save_then_open_round_trips, tilted_ucs_places_planar_entities_with_the_plane_normal).
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