diff -r 000000000000 -r 6474c204b198 mobile/android/base/gfx/SubdocumentScrollHelper.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mobile/android/base/gfx/SubdocumentScrollHelper.java	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,148 @@
+/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-
+ * 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/. */
+
+package org.mozilla.gecko.gfx;
+
+import org.mozilla.gecko.GeckoAppShell;
+import org.mozilla.gecko.GeckoEvent;
+import org.mozilla.gecko.EventDispatcher;
+import org.mozilla.gecko.util.GeckoEventListener;
+
+import org.json.JSONException;
+import org.json.JSONObject;
+
+import android.graphics.PointF;
+import android.os.Handler;
+import android.util.Log;
+
+class SubdocumentScrollHelper implements GeckoEventListener {
+    private static final String LOGTAG = "GeckoSubdocScroll";
+
+    private static String MESSAGE_PANNING_OVERRIDE = "Panning:Override";
+    private static String MESSAGE_CANCEL_OVERRIDE = "Panning:CancelOverride";
+    private static String MESSAGE_SCROLL = "Gesture:Scroll";
+    private static String MESSAGE_SCROLL_ACK = "Gesture:ScrollAck";
+
+    private final Handler mUiHandler;
+    private final EventDispatcher mEventDispatcher;
+
+    /* This is the amount of displacement we have accepted but not yet sent to JS; this is
+     * only valid when mOverrideScrollPending is true. */
+    private final PointF mPendingDisplacement;
+
+    /* When this is true, we're sending scroll events to JS to scroll the active subdocument. */
+    private boolean mOverridePanning;
+
+    /* When this is true, we have received an ack for the last scroll event we sent to JS, and
+     * are ready to send the next scroll event. Note we only ever have one scroll event inflight
+     * at a time. */
+    private boolean mOverrideScrollAck;
+
+    /* When this is true, we have a pending scroll that we need to send to JS; we were unable
+     * to send it when it was initially requested because mOverrideScrollAck was not true. */
+    private boolean mOverrideScrollPending;
+
+    /* When this is true, the last scroll event we sent actually did some amount of scrolling on
+     * the subdocument; we use this to decide when we have reached the end of the subdocument. */
+    private boolean mScrollSucceeded;
+
+    SubdocumentScrollHelper(EventDispatcher eventDispatcher) {
+        // mUiHandler will be bound to the UI thread since that's where this constructor runs
+        mUiHandler = new Handler();
+        mPendingDisplacement = new PointF();
+
+        mEventDispatcher = eventDispatcher;
+        registerEventListener(MESSAGE_PANNING_OVERRIDE);
+        registerEventListener(MESSAGE_CANCEL_OVERRIDE);
+        registerEventListener(MESSAGE_SCROLL_ACK);
+    }
+
+    void destroy() {
+        unregisterEventListener(MESSAGE_PANNING_OVERRIDE);
+        unregisterEventListener(MESSAGE_CANCEL_OVERRIDE);
+        unregisterEventListener(MESSAGE_SCROLL_ACK);
+    }
+
+    private void registerEventListener(String event) {
+        mEventDispatcher.registerEventListener(event, this);
+    }
+
+    private void unregisterEventListener(String event) {
+        mEventDispatcher.unregisterEventListener(event, this);
+    }
+
+    boolean scrollBy(PointF displacement) {
+        if (! mOverridePanning) {
+            return false;
+        }
+
+        if (! mOverrideScrollAck) {
+            mOverrideScrollPending = true;
+            mPendingDisplacement.x += displacement.x;
+            mPendingDisplacement.y += displacement.y;
+            return true;
+        }
+
+        JSONObject json = new JSONObject();
+        try {
+            json.put("x", displacement.x);
+            json.put("y", displacement.y);
+        } catch (JSONException e) {
+            Log.e(LOGTAG, "Error forming subwindow scroll message: ", e);
+        }
+        GeckoAppShell.sendEventToGecko(GeckoEvent.createBroadcastEvent(MESSAGE_SCROLL, json.toString()));
+
+        mOverrideScrollAck = false;
+        mOverrideScrollPending = false;
+        // clear the |mPendingDisplacement| after serializing |displacement| to
+        // JSON because they might be the same object
+        mPendingDisplacement.x = 0;
+        mPendingDisplacement.y = 0;
+
+        return true;
+    }
+
+    void cancel() {
+        mOverridePanning = false;
+    }
+
+    boolean scrolling() {
+        return mOverridePanning;
+    }
+
+    boolean lastScrollSucceeded() {
+        return mScrollSucceeded;
+    }
+
+    // GeckoEventListener implementation
+
+    @Override
+    public void handleMessage(final String event, final JSONObject message) {
+        // This comes in on the Gecko thread; hand off the handling to the UI thread.
+        mUiHandler.post(new Runnable() {
+            @Override
+            public void run() {
+                try {
+                    if (MESSAGE_PANNING_OVERRIDE.equals(event)) {
+                        mOverridePanning = true;
+                        mOverrideScrollAck = true;
+                        mOverrideScrollPending = false;
+                        mScrollSucceeded = true;
+                    } else if (MESSAGE_CANCEL_OVERRIDE.equals(event)) {
+                        mOverridePanning = false;
+                    } else if (MESSAGE_SCROLL_ACK.equals(event)) {
+                        mOverrideScrollAck = true;
+                        mScrollSucceeded = message.getBoolean("scrolled");
+                        if (mOverridePanning && mOverrideScrollPending) {
+                            scrollBy(mPendingDisplacement);
+                        }
+                    }
+                } catch (Exception e) {
+                    Log.e(LOGTAG, "Exception handling message", e);
+                }
+            }
+        });
+    }
+}