Tree: show indicator in drop node (#12854)

packages/tree/src/tree.vue

@@ -420,14 +420,15 @@
           dropType = 'none';
         }
 
+        const iconPosition = dropNode.$el.querySelector('.el-tree-node__expand-icon').getBoundingClientRect();
         const dropIndicator = this.$refs.dropIndicator;
         if (dropType === 'before') {
-          indicatorTop = targetPosition.top - treePosition.top;
+          indicatorTop = iconPosition.top - treePosition.top;
         } else if (dropType === 'after') {
-          indicatorTop = targetPosition.bottom - treePosition.top;
+          indicatorTop = iconPosition.bottom - treePosition.top;
         }
         dropIndicator.style.top = indicatorTop + 'px';
-        dropIndicator.style.left = (targetPosition.right - treePosition.left) + 'px';
+        dropIndicator.style.left = (iconPosition.right - treePosition.left) + 'px';
 
         if (dropType === 'inner') {
           addClass(dropNode.$el, 'is-drop-inner');