fix(商品详情): 回退页面高度为窗口高度

当页面回退时,pageScrollHeight高度为0,此时使用app页面高度做fallback

 export default {
+  deactivated() {
+    this.preVisibleEls = null;
+  },
   methods: {
     isVisible(offsetInfo, viewInfo, ratio = 0.5) {
       return !((offsetInfo.offsetTop + offsetInfo.offsetHeight * (1 - ratio)) < viewInfo.top ||
@@ -7,7 +10,22 @@ export default {
     scrollEndHandler({y = this.$refs.pageScroll && this.$refs.pageScroll.$el.offsetTop} = {}) {
       const scrollTop = Math.abs(y);
 
-      const pageScrollHeight = this.$refs.pageScroll.$el.offsetHeight;
+      let pageScrollHeight = this.$refs.pageScroll.$el.offsetHeight;
+
+      if (pageScrollHeight === 0) {
+        const app = document.querySelector('#app');
+        const header = document.querySelector('.layout-header');
+        const footer = document.querySelector('.footer');
+
+        pageScrollHeight = app && app.offsetHeight || window.innerHeight;
+        if (header) {
+          pageScrollHeight -= header.offsetHeight;
+        }
+        if (footer) {
+          pageScrollHeight -= footer.offsetHeight;
+        }
+      }
+
       const viewInfo = {
         top: scrollTop,
         bottom: scrollTop + pageScrollHeight,