Bug 938189 - Avoid duplicate snapshot taking and compression when using history swipe animations. r=felipe

browser/base/content/browser-gestureSupport.js

@@ -741,9 +741,13 @@ let gHistorySwipeAnimation = {
         break;
       case "pagehide":
         if (aEvent.target == gBrowser.selectedBrowser.contentDocument) {
-          // Take a snapshot of a page whenever it's about to be navigated away
-          // from.
-          this._takeSnapshot();
+          // Take and compress a snapshot of a page whenever it's about to be
+          // navigated away from. We already have a snapshot of the page if an
+          // animation is running, so we're left with compressing it.
+          if (!this.isAnimationRunning()) {
+            this._takeSnapshot();
+          }
+          this._compressSnapshotAtCurrentIndex();
         }
         break;
     }
@@ -935,13 +939,27 @@ let gHistorySwipeAnimation = {
     aBox.style.transform = transform;
   },
 
+  /**
+   * Verifies that we're ready to take snapshots based on the global pref and
+   * the current index in history.
+   *
+   * @return true if we're ready to take snapshots, false otherwise.
+   */
+  _readyToTakeSnapshots: function HSA__readyToTakeSnapshots() {
+    if ((this._maxSnapshots < 1) ||
+        (gBrowser.webNavigation.sessionHistory.index < 0)) {
+      return false;
+    }
+    return true;
+  },
+
   /**
    * Takes a snapshot of the page the browser is currently on.
    */
   _takeSnapshot: function HSA__takeSnapshot() {
-    if ((this._maxSnapshots < 1) ||
-        (gBrowser.webNavigation.sessionHistory.index < 0))
+    if (!this._readyToTakeSnapshots()) {
       return;
+    }
 
     let canvas = null;
 
@@ -1010,9 +1028,27 @@ let gHistorySwipeAnimation = {
       image: aCanvas,
       scale: window.devicePixelRatio
     };
+  },
+
+  /**
+   * Compresses the HTMLCanvasElement that's stored at the current history
+   * index in the snapshot array and stores the compressed image in its place.
+   */
+  _compressSnapshotAtCurrentIndex:
+  function HSA__compressSnapshotAtCurrentIndex() {
+    if (!this._readyToTakeSnapshots()) {
+      // We didn't take a snapshot earlier because we weren't ready to, so
+      // there's nothing to compress.
+      return;
+    }
+
+    let browser = gBrowser.selectedBrowser;
+    let snapshots = browser.snapshots;
+    let currIndex = browser.webNavigation.sessionHistory.index;
 
     // Kick off snapshot compression.
-    aCanvas.toBlob(function(aBlob) {
+    let canvas = snapshots[currIndex].image;
+    canvas.toBlob(function(aBlob) {
         if (snapshots[currIndex]) {
           snapshots[currIndex].image = aBlob;
         }