diff --git a/app/src/renderer/src/components/FlapNumber.vue b/app/src/renderer/src/components/FlapNumber.vue
new file mode 100644
index 0000000..ae8dd0b
--- /dev/null
+++ b/app/src/renderer/src/components/FlapNumber.vue
@@ -0,0 +1,151 @@
+<script setup>
+import { computed, onUnmounted, ref, watch } from 'vue';
+import {
+  createFlapState,
+  finishFlap,
+  flapSlots,
+  requestFlap,
+} from '../flap-number.mjs';
+
+const props = defineProps({
+  value: { type: [Number, String], required: true },
+});
+
+const state = ref(createFlapState(props.value));
+const motionQuery = typeof window !== 'undefined'
+  ? window.matchMedia('(prefers-reduced-motion: reduce)')
+  : null;
+let reducedMotion = Boolean(motionQuery?.matches);
+let completionVersion = state.value.version;
+const completedSlots = new Set();
+
+const slots = computed(() => state.value.animating
+  ? flapSlots(state.value.from, state.value.to)
+  : flapSlots(state.value.settled, state.value.settled));
+
+function resetCompletionTracking() {
+  completionVersion = state.value.version;
+  completedSlots.clear();
+}
+
+function applyValue(value) {
+  const previousVersion = state.value.version;
+  state.value = requestFlap(state.value, value, { reducedMotion });
+  if (state.value.version !== previousVersion || !state.value.animating) resetCompletionTracking();
+}
+
+function handleFlapEnd(index) {
+  if (!state.value.animating) return;
+  if (completionVersion !== state.value.version) resetCompletionTracking();
+  completedSlots.add(index);
+  const changedSlots = slots.value.filter(slot => slot.changed).length;
+  if (completedSlots.size < changedSlots) return;
+  state.value = finishFlap(state.value);
+  resetCompletionTracking();
+}
+
+function handleMotionPreference(event) {
+  reducedMotion = event.matches;
+  applyValue(props.value);
+}
+
+watch(() => props.value, applyValue);
+motionQuery?.addEventListener?.('change', handleMotionPreference);
+
+onUnmounted(() => {
+  motionQuery?.removeEventListener?.('change', handleMotionPreference);
+});
+</script>
+
+<template>
+  <span class="flap-number" :aria-label="String(value)">
+    <span
+      v-for="(slot, index) in slots"
+      :key="`${state.version}:${index}`"
+      class="flap-slot"
+      :class="{ flipping: state.animating && slot.changed }"
+      aria-hidden="true"
+    >
+      <span v-if="!state.animating || !slot.changed" class="flap-digit stable">{{ slot.to }}</span>
+      <template v-else>
+        <span class="flap-digit flap-new-top">{{ slot.to }}</span>
+        <span class="flap-digit flap-old-bottom">{{ slot.from }}</span>
+        <span class="flap-digit flap-old-top">{{ slot.from }}</span>
+        <span class="flap-digit flap-new-bottom" @animationend.stop="handleFlapEnd(index)">{{ slot.to }}</span>
+      </template>
+    </span>
+  </span>
+</template>
+
+<style scoped>
+.flap-number {
+  display: inline-flex;
+  align-items: center;
+  height: 1lh;
+  line-height: inherit;
+  vertical-align: middle;
+  white-space: nowrap;
+}
+
+.flap-slot {
+  position: relative;
+  width: 1ch;
+  height: 1lh;
+  overflow: hidden;
+  contain: paint;
+}
+
+.flap-digit {
+  position: absolute;
+  inset: 0;
+  display: grid;
+  place-items: center;
+  backface-visibility: hidden;
+}
+
+.flap-slot.flipping {
+  perspective: 90px;
+  transform-style: preserve-3d;
+}
+
+.flap-slot.flipping::after {
+  content: '';
+  position: absolute;
+  z-index: 6;
+  top: 50%;
+  left: 12%;
+  right: 12%;
+  height: 1px;
+  background: rgba(255, 255, 255, .09);
+  pointer-events: none;
+}
+
+.flap-new-top { z-index: 1; clip-path: inset(0 0 50% 0); }
+.flap-old-bottom { z-index: 1; clip-path: inset(50% 0 0 0); }
+.flap-old-top {
+  z-index: 4;
+  clip-path: inset(0 0 50% 0);
+  transform-origin: center 50%;
+  animation: flap-top-out 110ms cubic-bezier(.55, 0, 1, .45) both;
+}
+.flap-new-bottom {
+  z-index: 5;
+  clip-path: inset(50% 0 0 0);
+  transform-origin: center 50%;
+  animation: flap-bottom-in 120ms cubic-bezier(.22, 1, .36, 1) 100ms both;
+}
+
+@keyframes flap-top-out {
+  from { transform: rotateX(0); }
+  to { transform: rotateX(-90deg); }
+}
+
+@keyframes flap-bottom-in {
+  from { transform: rotateX(90deg); }
+  to { transform: rotateX(0); }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .flap-digit { animation: none !important; }
+}
+</style>
diff --git a/app/src/renderer/src/flap-number.mjs b/app/src/renderer/src/flap-number.mjs
new file mode 100644
index 0000000..b3ae09d
--- /dev/null
+++ b/app/src/renderer/src/flap-number.mjs
@@ -0,0 +1,61 @@
+function normalizeFlapValue(value) {
+  return String(value ?? '');
+}
+
+export function createFlapState(value) {
+  const settled = normalizeFlapValue(value);
+  return {
+    settled,
+    from: settled,
+    to: settled,
+    animating: false,
+    queued: null,
+    version: 0,
+  };
+}
+
+export function requestFlap(state, value, { reducedMotion = false } = {}) {
+  const next = normalizeFlapValue(value);
+  if (reducedMotion) return createFlapState(next);
+  if (state.animating) {
+    if (next === state.to) return { ...state, queued: null };
+    return { ...state, queued: next };
+  }
+  if (next === state.settled) return state;
+  return {
+    ...state,
+    from: state.settled,
+    to: next,
+    animating: true,
+    queued: null,
+    version: state.version + 1,
+  };
+}
+
+export function finishFlap(state) {
+  if (!state.animating) return state;
+  const settled = state.to;
+  const queued = state.queued;
+  const stable = {
+    ...state,
+    settled,
+    from: settled,
+    to: settled,
+    animating: false,
+    queued: null,
+  };
+  return queued !== null && queued !== settled ? requestFlap(stable, queued) : stable;
+}
+
+export function flapSlots(fromValue, toValue) {
+  const from = normalizeFlapValue(fromValue);
+  const to = normalizeFlapValue(toValue);
+  const width = Math.max(from.length, to.length);
+  const oldText = from.padStart(width, ' ');
+  const newText = to.padStart(width, ' ');
+  return Array.from({ length: width }, (_, index) => ({
+    from: oldText[index],
+    to: newText[index],
+    changed: oldText[index] !== newText[index],
+  }));
+}
diff --git a/app/src/renderer/src/views/SessionDetail.vue b/app/src/renderer/src/views/SessionDetail.vue
index 85e3ca6..cf6d027 100644
--- a/app/src/renderer/src/views/SessionDetail.vue
+++ b/app/src/renderer/src/views/SessionDetail.vue
@@ -4,6 +4,7 @@ import { useRouter, useRoute } from 'vue-router';
 import { state, FOLDER_SVG } from '../store.js';
 import { loadSessionDetail, isTextTruncated, loadFullText } from '../data.js';
 import { clearSessionDirty, consumeGlobalSessionDirty } from '../session-live.mjs';
+import FlapNumber from '../components/FlapNumber.vue';
 import {
   escapeHTML,
   fmtRelative,
@@ -954,7 +955,7 @@ function getToolCallParsedInput(tc) {
       <button class="msg-nav-btn" @click="navTo('prev')" :disabled="currentMsgIdx === 0" title="Previous">
         <svg viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><path d="M10 4l-4 4 4 4"/></svg>
       </button>
-      <span class="msg-nav-pos"><span class="msg-nav-current">{{ currentMsgIdx + 1 }}</span> / {{ totalMsgs }}</span>
+      <span class="msg-nav-pos"><span class="msg-nav-current">{{ currentMsgIdx + 1 }}</span> / <FlapNumber :value="totalMsgs" /></span>
       <button class="msg-nav-btn" @click="navTo('next')" :disabled="currentMsgIdx >= totalMsgs - 1" title="Next">
         <svg viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><path d="M6 4l4 4-4 4"/></svg>
       </button>
diff --git a/tests/flap-number.test.mjs b/tests/flap-number.test.mjs
new file mode 100644
index 0000000..63355ac
--- /dev/null
+++ b/tests/flap-number.test.mjs
@@ -0,0 +1,84 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+
+import {
+  createFlapState,
+  finishFlap,
+  flapSlots,
+  requestFlap,
+} from '../app/src/renderer/src/flap-number.mjs';
+
+test('flap slots animate only digits that changed', () => {
+  assert.deepEqual(flapSlots(822, 823), [
+    { from: '8', to: '8', changed: false },
+    { from: '2', to: '2', changed: false },
+    { from: '2', to: '3', changed: true },
+  ]);
+});
+
+test('flap state queues rapid updates without interrupting the active flap', () => {
+  let state = createFlapState(822);
+  state = requestFlap(state, 823);
+  state = requestFlap(state, 824);
+
+  assert.equal(state.from, '822');
+  assert.equal(state.to, '823');
+  assert.equal(state.queued, '824');
+
+  state = finishFlap(state);
+  assert.equal(state.from, '823');
+  assert.equal(state.to, '824');
+  assert.equal(state.animating, true);
+
+  state = finishFlap(state);
+  assert.equal(state.settled, '824');
+  assert.equal(state.animating, false);
+});
+
+test('the latest request clears an older queued value when it matches the active target', () => {
+  let state = createFlapState(822);
+  state = requestFlap(state, 823);
+  state = requestFlap(state, 824);
+  state = requestFlap(state, 823);
+
+  assert.equal(state.to, '823');
+  assert.equal(state.queued, null);
+  state = finishFlap(state);
+  assert.equal(state.settled, '823');
+  assert.equal(state.animating, false);
+});
+
+test('reduced motion updates the settled value immediately', () => {
+  const state = requestFlap(createFlapState(822), 823, { reducedMotion: true });
+
+  assert.equal(state.settled, '823');
+  assert.equal(state.animating, false);
+});
+
+test('queued empty string remains a valid latest value for the generic component', () => {
+  let state = createFlapState('1');
+  state = requestFlap(state, '2');
+  state = requestFlap(state, '');
+  state = finishFlap(state);
+
+  assert.equal(state.to, '');
+  assert.equal(state.animating, true);
+});
+
+test('SessionDetail applies flap motion only to the total message count', () => {
+  const source = readFileSync(new URL('../app/src/renderer/src/views/SessionDetail.vue', import.meta.url), 'utf8');
+
+  assert.match(source, /<FlapNumber\s+:value="totalMsgs"/);
+  assert.match(source, /msg-nav-current[^>]*>\{\{ currentMsgIdx \+ 1 \}\}/);
+});
+
+test('FlapNumber shares one relative geometry and finishes from animationend', () => {
+  const source = readFileSync(new URL('../app/src/renderer/src/components/FlapNumber.vue', import.meta.url), 'utf8');
+
+  assert.match(source, /width:\s*1ch/);
+  assert.match(source, /height:\s*1lh/);
+  assert.match(source, /@animationend\.stop=/);
+  assert.doesNotMatch(source, /height:\s*15px/);
+  assert.doesNotMatch(source, /setTimeout/);
+});