fix(promotion merch detail): 修复商品详情页下拉错位问题并添加商品描述省略

1.  添加商品描述文本的多行省略显示样式
2.  动态计算顶部header高度替代固定值，解决页面滚动错位问题
3.  监听merch数据变化重新计算头部高度

pages_promotion/merch/detail.vue

@@ -23,6 +23,7 @@
 						<text class="icox icox-xing"></text>
 						<text class="icox icox-xing"></text>
 					</view>
+					<view style="overflow:hidden;text-overflow:ellipsis;display:-webkit-box;-webkit-line-clamp:2;-webkit-box-orient:vertical;">{{ merch.desc }}</view>
 				</view>
 
 			</view>
@@ -58,7 +59,7 @@
 			</view>
 		</view>
 
-		<mescroll-uni top="240" ref="mescroll" @getData="getGoodsList">
+		<mescroll-uni :top="headerHeight" ref="mescroll" @getData="getGoodsList">
 			<block slot="list">
 				<view class="goods-list single-column" :class="{ show: isList }">
 					<view class="goods-item margin-bottom" v-for="(item, index) in goodsList" :key="index"
@@ -319,9 +320,37 @@ import list from './public/js/list.js';
 
 export default {
 	data() {
-		return {};
+		return {
+			headerHeight: 320,
+			screenWidth: uni.getSystemInfoSync().windowWidth
+		};
 	},
-	mixins: [list]
+	mixins: [list],
+	watch: {
+		merch: {
+			handler() {
+				this.$nextTick(() => {
+					this.initHeaderHeight();
+				});
+			},
+			deep: true
+		}
+	},
+	methods: {
+		initHeaderHeight() {
+			setTimeout(() => {
+				const query = uni.createSelectorQuery().in(this);
+				query.select('.head-wrap').boundingClientRect(rect => {
+					if (rect) {
+						this.headerHeight = (rect.height / this.screenWidth) * 750;
+					}
+				}).exec();
+			}, 100);
+		}
+	},
+	mounted() {
+		this.initHeaderHeight();
+	}
 };
 </script>