fix selection loss when checking/unchecking list

Author: jhchen

formats/list.js

@@ -24,13 +24,15 @@ class ListItem extends Block {
   constructor(scroll, domNode) {
     super(scroll, domNode);
     const ui = domNode.ownerDocument.createElement('span');
-    const listEventHandler = () => {
+    const listEventHandler = e => {
       if (!scroll.isEnabled()) return;
       const format = this.statics.formats(domNode, scroll);
       if (format === 'checked') {
         this.format('list', 'unchecked');
+        e.preventDefault();
       } else if (format === 'unchecked') {
         this.format('list', 'checked');
+        e.preventDefault();
       }
     };
     ui.addEventListener('mousedown', listEventHandler);