style(viewer): 广告轮播切换柔化——全帧堆叠淡入淡出

堆叠帧常驻 DOM 预加载（首轮切换不空白），活跃帧独占交互/聚焦；
恢复 10:3 高度锚点（各帧自然高度不一致会让切换时容器跳动）。

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/viewer/src/__tests__/ad-carousel.spec.ts

@@ -31,7 +31,8 @@ function mountCarousel(ads: PublicAd[] = ADS): VueWrapper {
 }
 
 function currentSrc(wrapper: VueWrapper): string | null {
-  const img = wrapper.find('.ad-slide img').element as HTMLImageElement | undefined;
+  // 全帧堆叠后当前帧=活跃帧（2026-09-29 淡入淡出改造）。
+  const img = wrapper.find('.ad-slide.active img').element as HTMLImageElement | undefined;
   return img?.getAttribute('src') ?? null;
 }
 

apps/viewer/src/components/ads/AdCarousel.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, onBeforeUnmount, onMounted, ref, watch } from 'vue';
+import { onBeforeUnmount, onMounted, ref, watch } from 'vue';
 import { useI18n } from 'vue-i18n';
 import type { PublicAd } from '../../api/client';
 import { nextIndex, swipeStep } from '../../ads/carousel';
@@ -22,7 +22,6 @@ const emit = defineEmits<{ close: []; imageError: [adId: string] }>();
 const { t } = useI18n();
 
 const current = ref(0);
-const currentAd = computed(() => props.ads[current.value] ?? null);
 /** 指针悬停/按住期间暂停（恢复时重开完整 4s 窗口，非冻结剩余时长）。 */
 const paused = ref(false);
 
@@ -99,18 +98,25 @@ function openAd(ad: PublicAd): void {
     @pointerenter="paused = true" @pointerdown="paused = true" @pointerleave="paused = false"
     @touchstart="onTouchStart" @touchend="onTouchEnd"
   >
-    <button
-      v-if="currentAd !== null" type="button" class="ad-slide" :aria-label="t('ads.open')"
-      @click="openAd(currentAd)"
-    >
-      <img
-        :src="currentAd.imageUrl"
-        :alt="t('ads.label')"
-        loading="lazy"
-        draggable="false"
-        @error="currentAd !== null && emit('imageError', currentAd.adId)"
+    <!-- 全帧堆叠 + 透明度过渡（2026-09-29 切换柔化）：所有帧常驻 DOM 预加载
+         （首轮切换不空白），仅活跃帧可交互/可聚焦；错误上报只看活跃帧。 -->
+    <div class="ad-slides">
+      <button
+        v-for="(ad, i) in ads" :key="ad.adId" type="button" class="ad-slide"
+        :class="{ active: i === current }" :aria-label="t('ads.open')"
+        :aria-hidden="i === current ? undefined : 'true'"
+        :tabindex="i === current ? 0 : -1"
+        @click="openAd(ad)"
       >
-    </button>
+        <img
+          :src="ad.imageUrl"
+          :alt="t('ads.label')"
+          decoding="async"
+          draggable="false"
+          @error="i === current && emit('imageError', ad.adId)"
+        >
+      </button>
+    </div>
     <span class="ad-badge">{{ t('ads.label') }}</span>
     <button type="button" class="ad-close" :aria-label="t('ads.close')" @click="emit('close')">✕</button>
     <nav v-if="ads.length > 1" class="ad-dots" :aria-label="t('ads.dots')">
@@ -130,7 +136,8 @@ function openAd(ad: PublicAd): void {
 .ad-carousel {
   position: relative;
   width: 100%;
-  /* aspect-ratio: 10 / 3; 建议素材 1200×360 */
+  /* 堆叠帧的高度锚点（各帧自然高度不一致会让切换时容器跳动）；建议素材 1200×360 */
+  aspect-ratio: 10 / 3;
   margin: 16px 0;
   overflow: hidden;
   border-radius: var(--viewer-radius-img);
@@ -140,7 +147,14 @@ function openAd(ad: PublicAd): void {
    * ThumbnailStrip pan-x 的对偶（pan-y）。 */
   touch-action: pan-y;
 }
+.ad-slides {
+  position: relative;
+  width: 100%;
+  height: 100%;
+}
 .ad-slide {
+  position: absolute;
+  inset: 0;
   display: block;
   width: 100%;
   height: 100%;
@@ -148,6 +162,15 @@ function openAd(ad: PublicAd): void {
   border: 0;
   background: transparent;
   cursor: pointer;
+  /* 淡入淡出（切换柔化）：非活跃帧透明且不拦截交互；transition 只对 opacity，
+   * 与 prefers-reduced-motion 无关（纯视觉淡变，非位移动画）。 */
+  opacity: 0;
+  transition: opacity 0.45s ease;
+  pointer-events: none;
+}
+.ad-slide.active {
+  opacity: 1;
+  pointer-events: auto;
 }
 .ad-slide img {
   display: block;