style(Collapse): adjust border gag (#6361)

src/collapse-item/index.js

@@ -1,6 +1,5 @@
 // Utils
 import { createNamespace, isDef } from '../utils';
-import { BORDER_TOP } from '../utils/constant';
 import { raf, doubleRaf } from '../utils/dom/raf';
 
 // Mixins
@@ -173,7 +172,7 @@ export default createComponent({
 
   render() {
     return (
-      <div class={[bem(), { [BORDER_TOP]: this.index }]}>
+      <div class={[bem({ border: this.index })]}>
         {this.genTitle()}
         {this.genContent()}
       </div>

src/collapse-item/index.less

@@ -1,6 +1,15 @@
 @import '../style/var';
+@import '../style/mixins/hairline';
 
 .van-collapse-item {
+  position: relative;
+
+  &--border {
+    &::after {
+      .hairline-top(@cell-border-color, @padding-md, @padding-md);
+    }
+  }
+
   &__title {
     .van-cell__right-icon::before {
       transform: rotate(90deg);
@@ -8,7 +17,8 @@
     }
 
     &::after {
-      visibility: hidden;
+      right: @padding-md;
+      display: none;
     }
 
     &--expanded {
@@ -17,7 +27,7 @@
       }
 
       &::after {
-        visibility: visible;
+        display: block;
       }
     }