diff --git a/docs/dancer-asset.md b/docs/dancer-asset.md
new file mode 100644
index 0000000..5636b9b
--- /dev/null
+++ b/docs/dancer-asset.md
@@ -0,0 +1,10 @@
+# BusyWeek dancer asset
+
+`src/assets/busyweek-dancer.png` is an original 4×4 alarm-clock mascot sprite sheet generated for BusyWeek with OpenAI's built-in image generation tool on 2026-07-17. It does not copy the character or sprite from `Huxpro/lynx-pretext`.
+
+- Source dimensions after optimization: 768 × 768 RGBA PNG
+- Shipped size: approximately 416 KB
+- Animation cadence: 12 fps; one presentation is capped at four seconds
+- Redistribution: project-owned generated asset, distributed under BusyWeek's ISC license
+
+The solid chroma background was removed locally. The transparent sheet is translated inside a clipped viewport; no 60 fps video or per-frame asset decode is used.
diff --git a/src/App.css b/src/App.css
index e811587..81a54e7 100644
--- a/src/App.css
+++ b/src/App.css
@@ -107,6 +107,50 @@
   linear-direction: column;
   width: 100%;
 }
+.dancer-timeline-layer {
+  position: absolute;
+  top: 58px;
+  left: 50%;
+  width: 160px;
+  height: 160px;
+  margin-left: -80px;
+  z-index: 4;
+  pointer-events: none;
+}
+.dancer-sprite-window {
+  position: relative;
+  width: 160px;
+  height: 160px;
+  overflow: hidden;
+}
+.dancer-sprite-sheet {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 640px;
+  height: 640px;
+  transform-origin: top left;
+}
+.dancer-todo-text { width: 100%; flex-shrink: 0; }
+.dancer-todo-band {
+  display: flex;
+  flex-direction: row;
+  justify-content: space-between;
+  width: 100%;
+  height: 20px;
+}
+.dancer-todo-line {
+  font-size: 15px;
+  line-height: 20px;
+  color: #2b2f33;
+  white-space: nowrap;
+  overflow: hidden;
+}
+.dancer-todo-line--right { text-align: right; }
+.dancer-todo-text.todo-text--done .dancer-todo-line {
+  color: #c2c9d0;
+  text-decoration: line-through;
+}
 .day-list {
   position: relative;
   width: 100%;
@@ -318,6 +362,7 @@
 }
 .todo-body:active .todo-text { opacity: 0.68; }
 .todo-text--done { color: #c2c9d0; text-decoration: line-through; }
+.todo-text--dancer-hidden { opacity: 0; }
 .todo-input {
   width: 100%;
   height: 100%;
diff --git a/src/App.vue b/src/App.vue
index 3f6f374..ec6a646 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -9,6 +9,8 @@ import {
 } from 'vue-lynx'
 import {
   clearTodoTextMeasurementCache,
+  type DancerCopyBand,
+  layoutDancerCopy,
   measureTodoText,
   supportsRendererLayoutCorrection,
 } from '@busyweek/text-layout-backend'
@@ -25,7 +27,11 @@ import {
 } from './globalEventBinding.js'
 import { createStarterTimeline } from './starterTimeline.js'
 import { loadTimeline, saveTimeline } from './store.js'
-import { createTimelineMotionLayout } from './timelineMotion.js'
+import {
+  DAY_GAP,
+  DAY_HEADER_HEIGHT,
+  createTimelineMotionLayout,
+} from './timelineMotion.js'
 import { keepTodoEditAboveKeyboard } from './todoKeyboardAvoidance.js'
 import {
   TODO_MIN_ROW_HEIGHT,
@@ -50,6 +56,15 @@ import {
 } from './util.js'
 import DatePickerSheet from './components/DatePickerSheet.vue'
 import DayPickerSheet from './components/DayPickerSheet.vue'
+import dancerSprite from './assets/busyweek-dancer.png'
+import {
+  DANCER_FPS,
+  DANCER_LOOP_MS,
+  createTapSequenceArbiter,
+  dancerFrameAt,
+  dancerIntervalsForText,
+  spriteTransform,
+} from './dancerEffect.js'
 
 type AppState = 'LIST' | 'INPUT'
 type TodoTextLayoutBinding = {
@@ -78,6 +93,13 @@ const composerSubmitLabel = computed(() =>
 // cross-platform pickers (built from Lynx primitives — work on web + native)
 const dayPickerOpen = ref(false)
 const datePickerOpen = ref(false)
+const dancerActive = ref(false)
+const dancerFrame = ref(0)
+const dancerPaused = ref(false)
+const prefersReducedMotion = ref(false)
+let dancerStartedAt = 0
+let dancerTimer: ReturnType<typeof setInterval> | undefined
+let dancerDismissTimer: ReturnType<typeof setTimeout> | undefined
 
 // soft-keyboard height (device-independent px), used to lift the composer's
 // bottom bar clear of the keyboard on native. The input element event works
@@ -193,6 +215,7 @@ function bindGlobalEvents() {
       listeners: [
         ['keyboardstatuschanged', onKeyboardStatus],
         [BUSYWEEK_TODO_LONG_PRESS_EVENT, onWebTodoLongPress],
+        ['appstatuschanged', onAppStatusChanged],
       ],
     })
   } catch {
@@ -207,9 +230,15 @@ onMounted(async () => {
   measureTodoWidthProbe()
   const stored = await loadTimeline()
   timeline.value = stored ?? createStarterTimeline(getTodayDate())
+  try {
+    const matcher = (globalThis as unknown as { matchMedia?: (query: string) => { matches: boolean } }).matchMedia?.('(prefers-reduced-motion: reduce)')
+    prefersReducedMotion.value = matcher?.matches === true
+  } catch { /* Native runtimes do not expose matchMedia. */ }
 })
 
 onUnmounted(() => {
+  stopDancer()
+  subtitleTapSequence.dispose()
   removeGlobalEventListeners?.()
   lastTodoLayoutHeights.clear()
   clearTodoTextMeasurementCache()
@@ -314,6 +343,72 @@ const emptyHint = computed(() =>
     ? '打开右上角查看已完成'
     : '点右下角 + 添加事项吧',
 )
+const dancerSpriteStyle = computed(() => ({ transform: spriteTransform(dancerFrame.value) }))
+type DancerTodoLayout = {
+  bands: DancerCopyBand[]
+  intervals: readonly { left: number; right: number }[]
+}
+const dancerTodoLayouts = computed(() => {
+  const layouts: Record<string, DancerTodoLayout> = {}
+  if (!dancerActive.value) return layouts
+  for (const day of visibleDays.value) {
+    const dayLayout = motionLayout.value.days[day.key]
+    if (!dayLayout) continue
+    for (const todo of day.todos) {
+      const measurement = measureTodoText(todo.text, todoTextWidth.value)
+      const lineCount = measurement?.lineCount ?? 1
+      const rowTop = dayLayout.offset + DAY_GAP + DAY_HEADER_HEIGHT + (dayLayout.todoOffsets[todo.id] ?? 0)
+      const textTop = rowTop + 8
+      const geometry = dancerIntervalsForText(
+        textTop,
+        lineCount,
+        20,
+        dancerFrame.value,
+        todoTextWidth.value,
+      )
+      if (!geometry.affected) continue
+      layouts[todo.id] = {
+        intervals: geometry.intervals,
+        bands: layoutDancerCopy(todo.text, geometry.intervals, todoTextWidth.value),
+      }
+    }
+  }
+  return layouts
+})
+function dancerTodoLeftStyle(todoId: string, index: number) {
+  const left = dancerTodoLayouts.value[todoId]?.intervals[index]?.left ?? 1
+  return { width: `${left * 100}%` }
+}
+function dancerTodoRightStyle(todoId: string, index: number) {
+  const right = dancerTodoLayouts.value[todoId]?.intervals[index]?.right ?? 1
+  return { width: `${(1 - right) * 100}%` }
+}
+
+function stopDancer() {
+  dancerActive.value = false
+  if (dancerTimer) clearInterval(dancerTimer)
+  if (dancerDismissTimer) clearTimeout(dancerDismissTimer)
+  dancerTimer = undefined
+  dancerDismissTimer = undefined
+}
+function startDancer() {
+  if (dancerActive.value) { stopDancer(); return }
+  dancerActive.value = true
+  dancerStartedAt = Date.now()
+  dancerFrame.value = 0
+  if (!prefersReducedMotion.value) {
+    dancerTimer = setInterval(() => {
+      if (!dancerPaused.value) dancerFrame.value = dancerFrameAt(Date.now() - dancerStartedAt)
+    }, 1000 / DANCER_FPS)
+  }
+  dancerDismissTimer = setTimeout(stopDancer, DANCER_LOOP_MS)
+}
+function onAppStatusChanged(status: unknown) {
+  const value = typeof status === 'string' ? status : (status as { status?: unknown } | null)?.status
+  dancerPaused.value = value === 'background' || value === 'inactive' || value === 'hide'
+}
+const subtitleTapSequence = createTapSequenceArbiter(startDancer)
+function onSubtitleTap() { subtitleTapSequence.tap() }
 
 // --- helpers exposed to the template ---------------------------------------
 function isToday(dateStr: string): boolean {
@@ -743,7 +838,10 @@ function removeTodo(dayKey: string, id: string) {
       <view class="app-bar">
         <view class="brand">
           <text class="bw-text logo">BusyWeek!</text>
-          <text class="bw-text logo-accent">好忙啊</text>
+          <text
+            class="bw-text logo-accent"
+            @tap.stop="onSubtitleTap"
+          >好忙啊</text>
         </view>
         <view
           class="completed-toggle"
@@ -775,6 +873,16 @@ function removeTodo(dayKey: string, id: string) {
       :scroll-y="true"
     >
       <view class="timeline-content">
+      <view
+        v-if="dancerActive"
+        class="dancer-timeline-layer"
+        accessibility-label="BusyWeek 跳舞彩蛋"
+        user-interaction-enabled="false"
+      >
+        <view class="dancer-sprite-window" accessibility-element="false">
+          <image class="dancer-sprite-sheet" :src="dancerSprite" :style="dancerSpriteStyle" />
+        </view>
+      </view>
       <!-- Hidden exact-geometry row: its body is the real text column width,
            independent of screen width and responsive timeline sizing. -->
       <view
@@ -865,11 +973,34 @@ function removeTodo(dayKey: string, id: string) {
                 @tap.stop="startEdit(todo)"
                 @longpress.stop="openTodoEditor(day.key, todo)"
               >
+                <view
+                  v-if="editingId !== todo.id && dancerTodoLayouts[todo.id]"
+                  class="dancer-todo-text"
+                  :class="{ 'todo-text--done': todo.done }"
+                >
+                  <view
+                    v-for="(band, bandIndex) in dancerTodoLayouts[todo.id].bands"
+                    :key="bandIndex"
+                    class="dancer-todo-band"
+                  >
+                    <text
+                      class="bw-text dancer-todo-line"
+                      :style="dancerTodoLeftStyle(todo.id, bandIndex)"
+                    >{{ band.left?.text }}</text>
+                    <text
+                      class="bw-text dancer-todo-line dancer-todo-line--right"
+                      :style="dancerTodoRightStyle(todo.id, bandIndex)"
+                    >{{ band.right?.text }}</text>
+                  </view>
+                </view>
                 <text
                   v-if="editingId !== todo.id && supportsRendererLayoutCorrection"
                   class="bw-text todo-text"
                   :key="getTodoTextLayoutBinding(todo.id).key"
-                  :class="{ 'todo-text--done': todo.done }"
+                  :class="{
+                    'todo-text--done': todo.done,
+                    'todo-text--dancer-hidden': dancerTodoLayouts[todo.id],
+                  }"
                   @layout="getTodoTextLayoutBinding(todo.id).onLayout"
                   >{{ todo.text }}</text
                 >
@@ -877,7 +1008,10 @@ function removeTodo(dayKey: string, id: string) {
                   v-else-if="editingId !== todo.id"
                   class="bw-text todo-text"
                   :key="getTodoTextLayoutBinding(todo.id).key"
-                  :class="{ 'todo-text--done': todo.done }"
+                  :class="{
+                    'todo-text--done': todo.done,
+                    'todo-text--dancer-hidden': dancerTodoLayouts[todo.id],
+                  }"
                   >{{ todo.text }}</text
                 >
                 <textarea
diff --git a/src/assets/busyweek-dancer.png b/src/assets/busyweek-dancer.png
new file mode 100644
index 0000000..fae2406
Binary files /dev/null and b/src/assets/busyweek-dancer.png differ
diff --git a/src/dancerEffect.ts b/src/dancerEffect.ts
new file mode 100644
index 0000000..587445d
--- /dev/null
+++ b/src/dancerEffect.ts
@@ -0,0 +1,91 @@
+export const DANCER_FRAME_COUNT = 16
+export const DANCER_FPS = 12
+export const DANCER_LOOP_MS = 4_000
+export const DANCER_TAP_WINDOW_MS = 600
+
+export type DancerInterval = { left: number; right: number }
+export const DANCER_TIMELINE_TOP = 58
+export const DANCER_SIZE = 160
+
+// Each frame has six horizontal silhouette bands. Values are normalized to
+// the copy column, keeping the geometry deterministic on every viewport.
+const BASE_PROFILES: readonly (readonly DancerInterval[])[] = [
+  [{left:.39,right:.61},{left:.31,right:.69},{left:.27,right:.73},{left:.29,right:.71},{left:.36,right:.64},{left:.42,right:.58}],
+  [{left:.42,right:.60},{left:.34,right:.66},{left:.29,right:.70},{left:.32,right:.69},{left:.40,right:.66},{left:.45,right:.72}],
+  [{left:.40,right:.60},{left:.32,right:.68},{left:.25,right:.75},{left:.29,right:.71},{left:.34,right:.66},{left:.39,right:.61}],
+  [{left:.38,right:.62},{left:.29,right:.71},{left:.25,right:.74},{left:.31,right:.69},{left:.40,right:.65},{left:.47,right:.59}],
+]
+
+export function dancerFrameAt(elapsedMs: number, reducedMotion = false): number {
+  if (reducedMotion || !Number.isFinite(elapsedMs) || elapsedMs <= 0) return 0
+  return Math.floor(elapsedMs / (1000 / DANCER_FPS)) % DANCER_FRAME_COUNT
+}
+
+export function dancerProfile(frame: number): readonly DancerInterval[] {
+  const safeFrame = Number.isFinite(frame) ? Math.max(0, Math.floor(frame)) : 0
+  const base = BASE_PROFILES[safeFrame % BASE_PROFILES.length]
+  const drift = ((safeFrame % 8) - 3.5) * .008
+  return base.map(({ left, right }) => ({ left: left + drift, right: right + drift }))
+}
+
+export function dancerIntervalsForText(
+  textTop: number,
+  lineCount: number,
+  lineHeight: number,
+  frame: number,
+  textWidth = 320,
+): { affected: boolean; intervals: DancerInterval[] } {
+  const profile = dancerProfile(frame)
+  let affected = false
+  const intervals = Array.from({ length: Math.max(1, lineCount) }, (_, index) => {
+    const lineCenter = textTop + index * lineHeight + lineHeight / 2
+    const relativeY = lineCenter - DANCER_TIMELINE_TOP
+    if (relativeY < 0 || relativeY >= DANCER_SIZE) return { left: 1, right: 1 }
+    affected = true
+    const band = Math.min(
+      profile.length - 1,
+      Math.floor(relativeY / (DANCER_SIZE / profile.length)),
+    )
+    const interval = profile[band]
+    // Profiles are authored against the 320px timeline stage. Keep their
+    // silhouette width in pixels when the measured Todo body is narrower or
+    // wider, while preserving its center alignment with the dancer layer.
+    const scale = 320 / Math.max(1, textWidth)
+    return {
+      left: Math.max(0, .5 + (interval.left - .5) * scale),
+      right: Math.min(1, .5 + (interval.right - .5) * scale),
+    }
+  })
+  return { affected, intervals }
+}
+
+export function spriteTransform(frame: number): string {
+  const safe = ((Math.floor(frame) % DANCER_FRAME_COUNT) + DANCER_FRAME_COUNT) % DANCER_FRAME_COUNT
+  return `translate(${-25 * (safe % 4)}%, ${-25 * Math.floor(safe / 4)}%)`
+}
+
+export function createTapSequenceArbiter(
+  onWin: () => void,
+  requiredTaps = 2,
+  windowMs = DANCER_TAP_WINDOW_MS,
+  now: () => number = Date.now,
+) {
+  let tapCount = 0
+  let lastTapAt = Number.NEGATIVE_INFINITY
+  return {
+    tap() {
+      const tappedAt = now()
+      tapCount = tappedAt - lastTapAt <= windowMs ? tapCount + 1 : 1
+      lastTapAt = tappedAt
+      if (tapCount < requiredTaps) return false
+      tapCount = 0
+      lastTapAt = Number.NEGATIVE_INFINITY
+      onWin()
+      return true
+    },
+    dispose() {
+      tapCount = 0
+      lastTapAt = Number.NEGATIVE_INFINITY
+    },
+  }
+}
diff --git a/src/text-layout-backend.d.ts b/src/text-layout-backend.d.ts
index 81fcb6f..d79a40d 100644
--- a/src/text-layout-backend.d.ts
+++ b/src/text-layout-backend.d.ts
@@ -3,6 +3,8 @@ declare module '@busyweek/text-layout-backend' {
     lineCount: number
     textHeight: number
   }
+  export interface DancerCopyLine { text: string; width: number }
+  export interface DancerCopyBand { left: DancerCopyLine | null; right: DancerCopyLine | null }
 
   export const supportsRendererLayoutCorrection: boolean
 
@@ -12,6 +14,7 @@ declare module '@busyweek/text-layout-backend' {
   ): TodoTextMeasurement | null
 
   export function clearTodoTextMeasurementCache(): void
+  export function layoutDancerCopy(text: string, intervals: readonly { left: number; right: number }[], width: number): DancerCopyBand[]
 }
 
 declare namespace lynx {
diff --git a/src/textLayoutBackend.lynx.ts b/src/textLayoutBackend.lynx.ts
index b00a378..231b993 100644
--- a/src/textLayoutBackend.lynx.ts
+++ b/src/textLayoutBackend.lynx.ts
@@ -1,7 +1,11 @@
 import {
   clearCache as clearPretextCache,
   layout,
+  layoutNextLine,
   prepare,
+  prepareWithSegments,
+  type LayoutCursor,
+  type PreparedTextWithSegments,
   type PreparedText,
 } from 'lynx-pretext'
 
@@ -19,6 +23,7 @@ const WHITE_SPACE = 'pre-wrap' as const
 const LINE_HEIGHT = 20
 
 const preparedTextCache = new Map<string, PreparedText>()
+const dancerPreparedCache = new Map<string, PreparedTextWithSegments>()
 
 function createPreparationKey(text: string): string {
   return JSON.stringify([text, PLATFORM_FONT, WHITE_SPACE])
@@ -61,6 +66,7 @@ export function measureTodoText(
 
 export function clearTodoTextMeasurementCache(): void {
   preparedTextCache.clear()
+  dancerPreparedCache.clear()
 
   try {
     clearPretextCache()
@@ -68,3 +74,27 @@ export function clearTodoTextMeasurementCache(): void {
     // A missing native text capability must not make cache cleanup crash UI work.
   }
 }
+
+export interface DancerCopyLine { text: string; width: number }
+export interface DancerCopyBand { left: DancerCopyLine | null; right: DancerCopyLine | null }
+export function layoutDancerCopy(text: string, intervals: readonly { left: number; right: number }[], width: number): DancerCopyBand[] {
+  if (!text || width <= 0) return []
+  try {
+    let prepared = dancerPreparedCache.get(text)
+    if (!prepared) {
+      prepared = prepareWithSegments(text, PLATFORM_FONT, { whiteSpace: WHITE_SPACE })
+      dancerPreparedCache.set(text, prepared)
+    }
+    let cursor: LayoutCursor = { segmentIndex: 0, graphemeIndex: 0 }
+    const bands: DancerCopyBand[] = []
+    for (const interval of intervals) {
+      const left = layoutNextLine(prepared, cursor, Math.max(1, interval.left * width))
+      if (!left) break
+      cursor = left.end
+      const right = layoutNextLine(prepared, cursor, Math.max(1, (1 - interval.right) * width))
+      if (right) cursor = right.end
+      bands.push({ left: { text: left.text, width: left.width }, right: right ? { text: right.text, width: right.width } : null })
+    }
+    return bands
+  } catch { return [] }
+}
diff --git a/src/textLayoutBackend.web.ts b/src/textLayoutBackend.web.ts
index 14fc974..8ba0d05 100644
--- a/src/textLayoutBackend.web.ts
+++ b/src/textLayoutBackend.web.ts
@@ -1,7 +1,11 @@
 import {
   clearCache as clearPretextCache,
   layout,
+  layoutNextLine,
   prepare,
+  prepareWithSegments,
+  type LayoutCursor,
+  type PreparedTextWithSegments,
   type PreparedText,
 } from '@chenglou/pretext'
 
@@ -28,6 +32,7 @@ const WHITE_SPACE = 'pre-wrap' as const
 const LINE_HEIGHT = 20
 
 const preparedTextCache = new Map<string, PreparedText>()
+const dancerPreparedCache = new Map<string, PreparedTextWithSegments>()
 
 function supportsTextMeasurement(): boolean {
   const globals = globalThis as unknown as TextMeasurementGlobals
@@ -79,6 +84,7 @@ export function measureTodoText(
 
 export function clearTodoTextMeasurementCache(): void {
   preparedTextCache.clear()
+  dancerPreparedCache.clear()
 
   try {
     clearPretextCache()
@@ -86,3 +92,27 @@ export function clearTodoTextMeasurementCache(): void {
     // Browser capability changes must not make cache cleanup crash rendering.
   }
 }
+
+export interface DancerCopyLine { text: string; width: number }
+export interface DancerCopyBand { left: DancerCopyLine | null; right: DancerCopyLine | null }
+export function layoutDancerCopy(text: string, intervals: readonly { left: number; right: number }[], width: number): DancerCopyBand[] {
+  if (!supportsTextMeasurement() || !text || width <= 0) return []
+  try {
+    let prepared = dancerPreparedCache.get(text)
+    if (!prepared) {
+      prepared = prepareWithSegments(text, PLATFORM_FONT, { whiteSpace: WHITE_SPACE })
+      dancerPreparedCache.set(text, prepared)
+    }
+    let cursor: LayoutCursor = { segmentIndex: 0, graphemeIndex: 0 }
+    const bands: DancerCopyBand[] = []
+    for (const interval of intervals) {
+      const left = layoutNextLine(prepared, cursor, Math.max(1, interval.left * width))
+      if (!left) break
+      cursor = left.end
+      const right = layoutNextLine(prepared, cursor, Math.max(1, (1 - interval.right) * width))
+      if (right) cursor = right.end
+      bands.push({ left: { text: left.text, width: left.width }, right: right ? { text: right.text, width: right.width } : null })
+    }
+    return bands
+  } catch { return [] }
+}
diff --git a/tests/dancer-effect.test.ts b/tests/dancer-effect.test.ts
new file mode 100644
index 0000000..28a9d89
--- /dev/null
+++ b/tests/dancer-effect.test.ts
@@ -0,0 +1,66 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+
+import {
+  DANCER_FRAME_COUNT,
+  createTapSequenceArbiter,
+  dancerFrameAt,
+  dancerIntervalsForText,
+  dancerProfile,
+  spriteTransform,
+} from '../src/dancerEffect.ts'
+
+test('frame selection follows 12fps cadence and wraps', () => {
+  assert.equal(dancerFrameAt(0), 0)
+  assert.equal(dancerFrameAt(84), 1)
+  assert.equal(dancerFrameAt((1000 / 12) * DANCER_FRAME_COUNT), 0)
+  assert.equal(dancerFrameAt(999, true), 0)
+})
+
+test('sprite transform maps every frame into the 4x4 sheet', () => {
+  assert.equal(spriteTransform(0), 'translate(0%, 0%)')
+  assert.equal(spriteTransform(5), 'translate(-25%, -25%)')
+  assert.equal(spriteTransform(15), 'translate(-75%, -75%)')
+})
+
+test('exclusion intervals remain ordered and within the copy column', () => {
+  for (let frame = 0; frame < DANCER_FRAME_COUNT; frame += 1) {
+    for (const interval of dancerProfile(frame)) {
+      assert.ok(interval.left > 0 && interval.left < interval.right)
+      assert.ok(interval.right < 1)
+    }
+  }
+})
+
+test('real Todo lines only receive exclusion geometry where the dancer overlaps', () => {
+  const above = dancerIntervalsForText(0, 1, 20, 0)
+  assert.equal(above.affected, false)
+  assert.deepEqual(above.intervals, [{ left: 1, right: 1 }])
+
+  const crossing = dancerIntervalsForText(28, 4, 20, 0)
+  assert.equal(crossing.affected, true)
+  assert.deepEqual(crossing.intervals[0], { left: 1, right: 1 })
+  assert.ok(crossing.intervals[1].left < crossing.intervals[1].right)
+
+  const narrow = dancerIntervalsForText(58, 1, 20, 0, 200)
+  assert.ok(narrow.intervals[0].left < crossing.intervals[1].left)
+  assert.ok(narrow.intervals[0].right > crossing.intervals[1].right)
+})
+
+test('tap arbitration requires two quick taps and resets after winning', () => {
+  let wins = 0
+  let time = 0
+  const arbiter = createTapSequenceArbiter(() => { wins += 1 }, 2, 600, () => time)
+  assert.equal(arbiter.tap(), false)
+  assert.equal(wins, 0)
+  time = 601
+  assert.equal(arbiter.tap(), false)
+  assert.equal(wins, 0)
+  time = 900
+  assert.equal(arbiter.tap(), true)
+  assert.equal(wins, 1)
+  time = 901
+  assert.equal(arbiter.tap(), false)
+  assert.equal(wins, 1)
+  arbiter.dispose()
+})