1 files changed, 103 insertions, 0 deletions
diff --git a/toolkit/modules/sessionstore/ScrollPosition.jsm b/toolkit/modules/sessionstore/ScrollPosition.jsm
new file mode 100644
index 000000000..5267f332a
--- /dev/null
+++ b/toolkit/modules/sessionstore/ScrollPosition.jsm
@@ -0,0 +1,103 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["ScrollPosition"];
+
+const Ci = Components.interfaces;
+
+/**
+ * It provides methods to collect scroll positions from single frames and to
+ * restore scroll positions for frame trees.
+ *
+ * This is a child process module.
+ */
+this.ScrollPosition = Object.freeze({
+  collect(frame) {
+    return ScrollPositionInternal.collect(frame);
+  },
+
+  restoreTree(root, data) {
+    ScrollPositionInternal.restoreTree(root, data);
+  }
+});
+
+/**
+ * This module's internal API.
+ */
+var ScrollPositionInternal = {
+  /**
+   * Collects scroll position data for any given |frame| in the frame hierarchy.
+   *
+   * @param frame (DOMWindow)
+   *
+   * @return {scroll: "x,y"} e.g. {scroll: "100,200"}
+   *         Returns null when there is no scroll data we want to store for the
+   *         given |frame|.
+   */
+  collect: function (frame) {
+    let ifreq = frame.QueryInterface(Ci.nsIInterfaceRequestor);
+    let utils = ifreq.getInterface(Ci.nsIDOMWindowUtils);
+    let scrollX = {}, scrollY = {};
+    utils.getScrollXY(false /* no layout flush */, scrollX, scrollY);
+
+    if (scrollX.value || scrollY.value) {
+      return {scroll: scrollX.value + "," + scrollY.value};
+    }
+
+    return null;
+  },
+
+  /**
+   * Restores scroll position data for any given |frame| in the frame hierarchy.
+   *
+   * @param frame (DOMWindow)
+   * @param value (object, see collect())
+   */
+  restore: function (frame, value) {
+    let match;
+
+    if (value && (match = /(\d+),(\d+)/.exec(value))) {
+      frame.scrollTo(match[1], match[2]);
+    }
+  },
+
+  /**
+   * Restores scroll position data for the current frame hierarchy starting at
+   * |root| using the given scroll position |data|.
+   *
+   * If the given |root| frame's hierarchy doesn't match that of the given
+   * |data| object we will silently discard data for unreachable frames. We
+   * may as well assign scroll positions to the wrong frames if some were
+   * reordered or removed.
+   *
+   * @param root (DOMWindow)
+   * @param data (object)
+   *        {
+   *          scroll: "100,200",
+   *          children: [
+   *            {scroll: "100,200"},
+   *            null,
+   *            {scroll: "200,300", children: [ ... ]}
+   *          ]
+   *        }
+   */
+  restoreTree: function (root, data) {
+    if (data.hasOwnProperty("scroll")) {
+      this.restore(root, data.scroll);
+    }
+
+    if (!data.hasOwnProperty("children")) {
+      return;
+    }
+
+    let frames = root.frames;
+    data.children.forEach((child, index) => {
+      if (child && index < frames.length) {
+        this.restoreTree(frames[index], child);
+      }
+    });
+  }
+};