bug(nav): Priority+ on /#/perf at ~1080px shows ONLY active pill — high-priority links vanish, More dropdown contains only active route (20th regression)

[Kpa-clawbot]

Symptom

Operator screenshot of staging at ~1080px-wide viewport on /#/perf: navbar shows ONLY the active-route pill ("Perf"). Brand + a few high-priority links should be visible inline; instead they're gone, AND clicking "More" only shows "Perf" (the active route) — not the missing links that should be there.

This is the 20th distinct nav-Priority+ overflow bug in the recent history (#345, #1097, #1102, #1104, #1106, #1109, #1139, #1148, #1163, #1311/#1312, …). The pattern is the same shape every time: at some narrow-ish viewport, the greedy fits() loop in public/app.js reaches a state where it either drops too much or drops nothing useful.

What's different vs prior fixes

• bug(top-nav): Priority+ overflow logic is half-built — CSS hides .is-overflow but NO JS adds the class. Nav links missing on medium-narrow viewports #1311/fix(nav): floor Priority+ overflow at high-priority links — fixes nav vanishing on non-high routes (#1311) #1312 (the most recent fix, merged 5 days ago) protected high-priority links from being DROPPED to overflow. The current bug is they're still all gone — different mechanism.
• The screenshot's More dropdown contains ONLY the active-route pill ("Perf"). The active route may be being added to overflowQueue first AND being the only one not being protected by the high-priority floor.
• Timing: bug observed AFTER tonight's burst of merges (fix(#1356): WCAG 2.2 AA map a11y — cluster bubbles, role pills, multi-byte labels #1357 / fix(#1360): cluster pill shows letter+count — restore count visibility regressed by #1357 #1362 / regression(map): #1362 pill max-width:4ch over-clamps multi-digit counts → 'R…' instead of 'R60' #1364 / fix(#1367): channels page chat-app redesign — restore prod row layout, drop analytics chip, add detail view #1376 / fix(#1361): theme customizer — colorblind presets [WIP] #1378 / fix(#1374): packet-route map modernized — role-aware markers, directional edges, WCAG 2.2 AA #1381). Most likely interaction: fix(#1361): theme customizer — colorblind presets [WIP] #1378 (CB presets) added new --mc-* CSS vars in :root that subtly changed link rendering widths, OR fix(#1367): channels page chat-app redesign — restore prod row layout, drop analytics chip, add detail view #1376 (channel chat-app) added new mobile CSS classes that interact with the nav breakpoint media queries.

Concrete repro

1. Open /#/perf on staging at viewport ~1080-1100 px wide.
2. Observe navbar — should show brand + high-priority links + More. Instead shows only "Perf" pill.
3. Click More — should list missing links. Instead lists only the active-route pill.

(Width threshold may vary by viewport — operator's screenshot is ~1080px from Firefox dev tools.)

Root cause hypothesis (to verify before fixing)

In public/app.js applyPriorityPlus():

• overflowQueue order matters. If the active-route pill (e.g. "Perf") is at position 0, the loop adds it first.
• The high-priority floor (if (overflowQueue[i].dataset.priority === 'high') break;) only breaks on the FIRST hit. If the floor breaks BEFORE adding any non-high link, More is populated with… nothing extra. Then a second pass somewhere ADDS just the active pill to More.
• The screenshot suggests overflow contains ONLY the active route pill, with no other links — which means inline got cleared but the floor wasn't honored.

Likely the bug is in the interplay between:

1. The greedy fits() loop (bug(top-nav): Priority+ overflow logic is half-built — CSS hides .is-overflow but NO JS adds the class. Nav links missing on medium-narrow viewports #1311 floor).
2. The "floor More at >=2 items" promotion (bug(nav): functional regressions reported in prod — hamburger inert + More ▾ shows only 1 item; not reproducible against master #1139 Bug B, line ~1238).
3. Some new measurement noise from tonight's CSS-var addition.

Diagnose via the existing E2E test test-nav-priority-1311-e2e.js — extend to cover the /perf route at 1080px viewport.

Acceptance criteria

• Reproduce locally at 1080×800 on /#/perf. Visible inline: brand + at least 3 high-priority links + More. NOT just "Perf" pill alone.
• Reproduce on at least one non-active high-priority route too (e.g. /#/audio-lab — if it's still around as a high-priority link).
• Active-route pill MUST always be visible inline (never overflowed to More) at any viewport.
• Any link in overflow that doesn't fit MUST still be reachable via the More dropdown.
• If the active route is itself NOT a high-priority link (e.g. /#/perf), the high-priority links must STILL be inline at any viewport ≥768px.
• E2E regression test: test-nav-priority-1390-e2e.js (or extension of 1311) — at 1080×800 on /#/perf, assert (a) ≥3 visible links inline, (b) More dropdown contains the LINKS that overflowed (not just active route), (c) every high-priority data-priority="high" link is visible inline OR in More.
• Mutation: revert the fix, assert the new E2E test fails.

Out of scope

• Redesigning Priority+ as a whole (it's been touched 20 times; the algorithm is the algorithm).
• Removing /perf from nav (it's a debug surface — not the navbar's fault).
• Mobile (<768px) — separate hamburger code path.

Refs

• bug(top-nav): Priority+ overflow logic is half-built — CSS hides .is-overflow but NO JS adds the class. Nav links missing on medium-narrow viewports #1311 / fix(nav): floor Priority+ overflow at high-priority links — fixes nav vanishing on non-high routes (#1311) #1312 — most recent prior fix (floor)
• bug(nav): functional regressions reported in prod — hamburger inert + More ▾ shows only 1 item; not reproducible against master #1139 / fix(nav): floor More menu at >=2 items (#1139 Bug B) #1148 — earlier floor at ≥2 items
• bug(nav): Priority+ collapses too aggressively at wide widths after #1097 — More menu also broken #1102 / fix(nav): #1102 — JS-driven Priority+ measures actual fit #1104 — JS-driven measure
• [#1050] Task 2: Nav fluid Priority+ adaptation at all widths #1055 / fix(nav): apply Priority+ at all widths (#1055) #1097 — apply at all widths
• public/app.js:1210-1245 — applyPriorityPlus() fits/floor loop
• public/style.css:1678-1729 — high-priority link contract