Introduce a button to toggle hiding/showing of the details in the events tab.

chrome/browser/resources/sync_internals/events.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function() {
-function makeLogEntryNode(entry) {
+'use strict';
+var toggleDisplay = function() {
+  var detailsNode = this.parentNode.getElementsByClassName('details')[0];
+  if (detailsNode.classList.contains('hidden')) {
+    detailsNode.classList.remove('hidden');
+  } else {
+    detailsNode.classList.add('hidden');
+  }
+}
+
+var makeLogEntryNode = function(entry) {
   var timeNode = document.createElement('td');
   timeNode.textContent = entry.date;
 
   var submoduleNode = document.createElement('td');
   submoduleNode.textContent = entry.submodule;
 
   var eventNode = document.createElement('td');
   eventNode.textContent = entry.event;
 
   var details = document.createElement('pre');
   details.textContent = JSON.stringify(entry.details, null, 2);
+  details.className = 'details';
+  details.classList.add('details');
+  details.classList.add('hidden');
+  var detailsToggleButton = document.createElement('button');
+  detailsToggleButton.addEventListener('click', toggleDisplay, false);
+  detailsToggleButton.textContent = 'Show/Hide Details';
   var detailsNode = document.createElement('td');
+  detailsNode.appendChild(detailsToggleButton);
   detailsNode.appendChild(details);
 
   var node = document.createElement('tr');
   node.appendChild(timeNode);
   node.appendChild(submoduleNode);
   node.appendChild(eventNode);
   node.appendChild(detailsNode);
 
   return node;
 }
 
-var syncEvents = document.getElementById('sync-events');
+var syncEvents = $('sync-events');
 
 var entries = chrome.sync.log.entries;
 for (var i = 0; i < entries.length; ++i) {
   syncEvents.appendChild(makeLogEntryNode(entries[i]));
 }
 
 chrome.sync.log.addEventListener('append', function(event) {
   syncEvents.appendChild(makeLogEntryNode(event.detail));
 });
 })();