[Reland] WebUI Tab Strip: Only track thumbnails for visible tabs

- Track thumbnails only if they are currently in view or are
just a standard finger swipe away.
- Do not track thumbnails for pinned tabs.
- Do not track thumbnails when the tab strip is closed.

Note that this CL does not implement actually untracking thumbnails.
It merely calls the method in the C++ side that will eventually do it.

Bug: 1015132
Change-Id: I16cce4a71ad3e6e887555be27e1525cdef9beb33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894245Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711609}

chrome/browser/resources/tab_strip/tab.js

@@ -137,10 +137,6 @@ export class TabElement extends CustomElement {
           this.toggleAttribute('has-alert-states_', alertIndicatorsCount > 0);
         });
 
-    if (!this.tab_ || this.tab_.id !== tab.id) {
-      this.tabsApi_.trackThumbnailForTab(tab.id);
-    }
-
     this.tab_ = Object.freeze(tab);
   }
 

chrome/browser/resources/tab_strip/tab_list.js

@@ -54,6 +54,24 @@ class TabListElement extends CustomElement {
      */
     this.draggedItem_;
 
+    /**
+     * An intersection observer is needed to observe which TabElements are
+     * currently in view or close to being in view, which will help determine
+     * which thumbnails need to be tracked to stay fresh and which can be
+     * untracked until they become visible.
+     * @private {!IntersectionObserver}
+     */
+    this.intersectionObserver_ = new IntersectionObserver(entries => {
+      for (const entry of entries) {
+        this.tabsApi_.setThumbnailTracked(
+            entry.target.tab.id, entry.isIntersecting);
+      }
+    }, {
+      // The horizontal root margin is set to 100% to also track thumbnails that
+      // are one standard finger swipe away.
+      rootMargin: '0% 100%',
+    });
+
     /** @private {!Element} */
     this.pinnedTabsContainerElement_ =
         /** @type {!Element} */ (
@@ -88,7 +106,7 @@ class TabListElement extends CustomElement {
     this.addEventListener(
         'dragover', (e) => this.onDragOver_(/** @type {!DragEvent} */ (e)));
     document.addEventListener(
-        'visibilitychange', () => this.moveOrScrollToActiveTab_());
+        'visibilitychange', () => this.onVisibilityChange_());
 
     if (loadTimeData.getBoolean('showDemoOptions')) {
       this.shadowRoot.querySelector('#demoOptions').style.display = 'block';
@@ -187,6 +205,8 @@ class TabListElement extends CustomElement {
    * @private
    */
   insertTabOrMoveTo_(tabElement, index) {
+    const isInserting = !tabElement.isConnected;
+
     // Remove the tabElement if it already exists in the DOM
     tabElement.remove();
 
@@ -201,6 +221,10 @@ class TabListElement extends CustomElement {
       this.tabsContainerElement_.insertBefore(
           tabElement, this.tabsContainerElement_.childNodes[offsetIndex]);
     }
+
+    if (isInserting) {
+      this.updateThumbnailTrackStatus_(tabElement);
+    }
   }
 
   /** @private */
@@ -319,7 +343,6 @@ class TabListElement extends CustomElement {
    */
   onTabCreated_(tab) {
     const tabElement = this.createTabElement_(tab);
-
     if (tabStripOptions.mruEnabled && tab.active && !tab.pinned &&
         tab.index !== this.pinnedTabsContainerElement_.childElementCount) {
       // Newly created active tabs should first be moved to the very beginning
@@ -383,9 +406,18 @@ class TabListElement extends CustomElement {
       if (tab.active) {
         this.scrollToTab_(tabElement);
       }
+
+      this.updateThumbnailTrackStatus_(tabElement);
     }
   }
 
+  /** @private */
+  onVisibilityChange_() {
+    this.moveOrScrollToActiveTab_();
+    Array.from(this.tabsContainerElement_.children)
+        .forEach((tabElement) => this.updateThumbnailTrackStatus_(tabElement));
+  }
+
   /**
    * @param {!TabElement} tabElement
    * @private
@@ -422,6 +454,24 @@ class TabListElement extends CustomElement {
       tab.updateThumbnail(imgData);
     }
   }
+
+  /**
+   * @param {!TabElement} tabElement
+   * @private
+   */
+  updateThumbnailTrackStatus_(tabElement) {
+    if (this.tabStripEmbedderProxy_.isVisible() && !tabElement.tab.pinned) {
+      // If the tab strip is visible and the tab is not pinned, let the
+      // IntersectionObserver start observing the TabElement to automatically
+      // determine if the tab's thumbnail should be tracked.
+      this.intersectionObserver_.observe(tabElement);
+    } else {
+      // If the tab strip is not visible or the tab is pinned, the tab does not
+      // need to show or update any thumbnails.
+      this.intersectionObserver_.unobserve(tabElement);
+      this.tabsApi_.setThumbnailTracked(tabElement.tab.id, false);
+    }
+  }
 }
 
 customElements.define('tabstrip-tab-list', TabListElement);

chrome/browser/resources/tab_strip/tabs_api_proxy.js

@@ -100,9 +100,10 @@ export class TabsApiProxy {
 
   /**
    * @param {number} tabId
+   * @param {boolean} thumbnailTracked
    */
-  trackThumbnailForTab(tabId) {
-    chrome.send('addTrackedTab', [tabId]);
+  setThumbnailTracked(tabId, thumbnailTracked) {
+    chrome.send('setThumbnailTracked', [tabId, thumbnailTracked]);
   }
 }
 

chrome/browser/ui/webui/tab_strip/tab_strip_ui.cc

@@ -217,10 +217,8 @@ class TabStripUIHandler : public content::WebUIMessageHandler,
         "getThemeColors", base::Bind(&TabStripUIHandler::HandleGetThemeColors,
                                      base::Unretained(this)));
     web_ui()->RegisterMessageCallback(
-        "addTrackedTab",
-        base::Bind(&TabStripUIHandler::AddTrackedTab, base::Unretained(this)));
-    web_ui()->RegisterMessageCallback(
-        "removeTrackedTab", base::Bind(&TabStripUIHandler::RemoveTrackedTab,
+        "setThumbnailTracked",
+        base::Bind(&TabStripUIHandler::HandleSetThumbnailTracked,
                    base::Unretained(this)));
     web_ui()->RegisterMessageCallback(
         "closeContainer", base::Bind(&TabStripUIHandler::HandleCloseContainer,
@@ -377,13 +375,15 @@ class TabStripUIHandler : public content::WebUIMessageHandler,
     ResolveJavascriptCallback(callback_id, layout);
   }
 
-  void AddTrackedTab(const base::ListValue* args) {
+  void HandleSetThumbnailTracked(const base::ListValue* args) {
     AllowJavascript();
 
     int tab_id = 0;
     if (!args->GetInteger(0, &tab_id))
       return;
 
+    const bool thumbnail_tracked = args->GetList()[1].GetBool();
+
     content::WebContents* tab = nullptr;
     if (!extensions::ExtensionTabUtil::GetTabById(tab_id, browser_->profile(),
                                                   true, &tab)) {
@@ -391,17 +391,13 @@ class TabStripUIHandler : public content::WebUIMessageHandler,
       DVLOG(1) << "Invalid tab ID";
       return;
     }
-    thumbnail_tracker_.WatchTab(tab);
-  }
-
-  void RemoveTrackedTab(const base::ListValue* args) {
-    AllowJavascript();
 
-    int tab_id = 0;
-    if (!args->GetInteger(0, &tab_id))
-      return;
+    if (thumbnail_tracked) {
+      thumbnail_tracker_.WatchTab(tab);
+    } else {
       // TODO(crbug.com/991393): un-watch tabs when we are done.
     }
+  }
 
   // Callback passed to |thumbnail_tracker_|. Called when a tab's thumbnail
   // changes, or when we start watching the tab.

chrome/test/data/webui/tab_strip/tab_list_test.js

@@ -61,6 +61,7 @@ suite('TabList', () => {
       alertStates: [],
       id: 0,
       index: 0,
+      pinned: false,
       title: 'Tab 1',
     },
     {
@@ -68,6 +69,7 @@ suite('TabList', () => {
       alertStates: [],
       id: 1,
       index: 1,
+      pinned: false,
       title: 'Tab 2',
     },
     {
@@ -75,6 +77,7 @@ suite('TabList', () => {
       alertStates: [],
       id: 2,
       index: 2,
+      pinned: false,
       title: 'Tab 3',
     },
   ];
@@ -85,6 +88,12 @@ suite('TabList', () => {
     webUIListenerCallback('tab-updated', updatedTab);
   }
 
+  function unpinTabAt(tab, index) {
+    const changeInfo = {index: index, pinned: false};
+    const updatedTab = Object.assign({}, tab, changeInfo);
+    webUIListenerCallback('tab-updated', updatedTab);
+  }
+
   function getUnpinnedTabs() {
     return tabList.shadowRoot.querySelectorAll('#tabsContainer tabstrip-tab');
   }
@@ -111,6 +120,7 @@ suite('TabList', () => {
       '--height': '100px',
       '--width': '150px',
     });
+    testTabStripEmbedderProxy.setVisible(true);
     TabStripEmbedderProxy.instance_ = testTabStripEmbedderProxy;
 
     tabList = document.createElement('tabstrip-tab-list');
@@ -404,4 +414,104 @@ suite('TabList', () => {
         assertEquals(moveId, tabs[1].id);
         assertEquals(newIndex, 0);
       });
+
+  test('tracks and untracks thumbnails based on viewport', async () => {
+    // Wait for slideIn animations to complete updating widths and reset
+    // resolvers to track new calls.
+    await tabList.animationPromises;
+    testTabsApiProxy.reset();
+    const tabElements = getUnpinnedTabs();
+
+    // Update width such that at most one tab can fit in the viewport at once.
+    tabList.style.setProperty('--tabstrip-tab-width', `${window.innerWidth}px`);
+
+    // At this point, the only visible tab should be the first tab. The second
+    // tab should fit within the rootMargin of the IntersectionObserver. The
+    // third tab should not be intersecting.
+    let [tabId, thumbnailTracked] =
+        await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(tabId, tabElements[2].tab.id);
+    assertEquals(thumbnailTracked, false);
+    assertEquals(testTabsApiProxy.getCallCount('setThumbnailTracked'), 1);
+    testTabsApiProxy.reset();
+
+    // Scroll such that the second tab is now the only visible tab. At this
+    // point, all 3 tabs should fit within the root and rootMargin of the
+    // IntersectionObserver. Since the 3rd tab was not being tracked before,
+    // it should be the only tab to become tracked.
+    document.documentElement.scrollLeft = tabElements[1].offsetLeft;
+    [tabId, thumbnailTracked] =
+        await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(tabId, tabElements[2].tab.id);
+    assertEquals(thumbnailTracked, true);
+    assertEquals(testTabsApiProxy.getCallCount('setThumbnailTracked'), 1);
+    testTabsApiProxy.reset();
+
+    // Scroll such that the third tab is now the only visible tab. At this
+    // point, the first tab should be outside of the rootMargin of the
+    // IntersectionObserver.
+    document.documentElement.scrollLeft = tabElements[2].offsetLeft;
+    [tabId, thumbnailTracked] =
+        await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(tabId, tabElements[0].tab.id);
+    assertEquals(thumbnailTracked, false);
+    assertEquals(testTabsApiProxy.getCallCount('setThumbnailTracked'), 1);
+  });
+
+  test('tracks and untracks thumbnails based on pinned state', async () => {
+    await tabList.animationPromises;
+    testTabsApiProxy.reset();
+
+    // Update width such that at all tabs can fit and do not fire the
+    // IntersectionObserver based on intersection alone.
+    tabList.style.setProperty(
+        '--tabstrip-tab-width', `${window.innerWidth / tabs.length}px`);
+
+    // Pinning the third tab should untrack thumbnails for the tab
+    pinTabAt(tabs[2], 0);
+    let [tabId, thumbnailTracked] =
+        await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(tabId, tabs[2].id);
+    assertEquals(thumbnailTracked, false);
+    testTabsApiProxy.reset();
+
+    // Unpinning the tab should re-track the thumbnails
+    unpinTabAt(tabs[2], 0);
+    [tabId, thumbnailTracked] =
+        await testTabsApiProxy.whenCalled('setThumbnailTracked');
+
+    // TODO(johntlee): Remove debug logs if tests are no longer flaky.
+    console.log(`Window width is ${window.innerWidth}px`);
+    for (const tabElement of getUnpinnedTabs()) {
+      console.log(`Tab ${tabElement.tab.id} is at ${tabElement.offsetLeft}`);
+    }
+
+    assertEquals(tabId, tabs[2].id);
+    assertEquals(thumbnailTracked, true);
+  });
+
+  test('should update thumbnail track status on visibilitychange', async () => {
+    await tabList.animationPromises;
+    testTabsApiProxy.reset();
+
+    testTabStripEmbedderProxy.setVisible(false);
+    document.dispatchEvent(new Event('visibilitychange'));
+
+    // The tab strip should force untrack thumbnails for all tabs.
+    await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(
+        testTabsApiProxy.getCallCount('setThumbnailTracked'), tabs.length);
+    testTabsApiProxy.reset();
+
+    // Update width such that at all tabs can fit
+    tabList.style.setProperty(
+        '--tabstrip-tab-width', `${window.innerWidth / tabs.length}px`);
+
+    testTabStripEmbedderProxy.setVisible(true);
+    document.dispatchEvent(new Event('visibilitychange'));
+
+    await testTabsApiProxy.whenCalled('setThumbnailTracked');
+    assertEquals(
+        testTabsApiProxy.getCallCount('setThumbnailTracked'), tabs.length);
+  });
 });

chrome/test/data/webui/tab_strip/tab_test.js

@@ -252,11 +252,7 @@ suite('Tab', function() {
     assertEquals(window.getComputedStyle(thumbnailImage).display, 'none');
   });
 
-  test('tracks and updates the thumbnail source', async () => {
-    const requestedTabId =
-        await testTabsApiProxy.whenCalled('trackThumbnailForTab');
-    assertEquals(requestedTabId, tab.id);
-
+  test('updates the thumbnail source', async () => {
     const thumbnailSource = 'data:mock-thumbnail-source';
     tabElement.updateThumbnail(thumbnailSource);
     assertEquals(

chrome/test/data/webui/tab_strip/test_tabs_api_proxy.js

@@ -11,7 +11,7 @@ export class TestTabsApiProxy extends TestBrowserProxy {
       'closeTab',
       'getTabs',
       'moveTab',
-      'trackThumbnailForTab',
+      'setThumbnailTracked',
     ]);
 
     this.tabs_;
@@ -41,7 +41,7 @@ export class TestTabsApiProxy extends TestBrowserProxy {
     this.tabs_ = tabs;
   }
 
-  trackThumbnailForTab(tabId) {
-    this.methodCalled('trackThumbnailForTab', tabId);
+  setThumbnailTracked(tabId, thumbnailTracked) {
+    this.methodCalled('setThumbnailTracked', [tabId, thumbnailTracked]);
   }
 }