diff -r 000000000000 -r 6474c204b198 browser/metro/base/content/apzc.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/metro/base/content/apzc.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,153 @@
+// -*- Mode: js2; tab-width: 2; indent-tabs-mode: nil; js2-basic-offset: 2; js2-skip-preprocessor-directives: t; -*-
+/* 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/. */
+
+let Cc = Components.classes;
+let Ci = Components.interfaces;
+let Cu = Components.utils;
+let Cr = Components.results;
+
+/**
+ * Misc. front end utilities for apzc management.
+ * the pref: layers.async-pan-zoom.enabled is true.
+ */
+
+var APZCObserver = {
+  _debugEvents: false,
+  _enabled: false,
+
+  get enabled() {
+    return this._enabled;
+  },
+
+  init: function() {
+    this._enabled = Services.prefs.getBoolPref(kAsyncPanZoomEnabled);
+    if (!this._enabled) {
+      return;
+    }
+
+    let os = Services.obs;
+    os.addObserver(this, "apzc-transform-begin", false);
+
+    Elements.tabList.addEventListener("TabSelect", this, true);
+    Elements.browsers.addEventListener("pageshow", this, true);
+    messageManager.addMessageListener("Content:ZoomToRect", this);
+  },
+
+  shutdown: function shutdown() {
+    if (!this._enabled) {
+      return;
+    }
+
+    let os = Services.obs;
+    os.removeObserver(this, "apzc-transform-begin");
+
+    Elements.tabList.removeEventListener("TabSelect", this, true);
+    Elements.browsers.removeEventListener("pageshow", this, true);
+    messageManager.removeMessageListener("Content:ZoomToRect", this);
+  },
+
+  handleEvent: function APZC_handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case 'TabSelect':
+        this._resetDisplayPort();
+        break;
+
+      case 'pageshow':
+        if (aEvent.target != Browser.selectedBrowser.contentDocument) {
+          break;
+        }
+        this._resetDisplayPort();
+        break;
+    }
+  },
+
+  observe: function ao_observe(aSubject, aTopic, aData) {
+    if (aTopic == "apzc-transform-begin") {
+      // When we're panning, hide the main scrollbars by setting imprecise
+      // input (which sets a property on the browser which hides the scrollbar
+      // via CSS).  This reduces jittering from left to right. We may be able
+      // to get rid of this once we implement axis locking in /gfx APZC.
+      if (InputSourceHelper.isPrecise) {
+        InputSourceHelper._imprecise();
+      }
+    }
+  },
+
+  receiveMessage: function(aMessage) {
+    let json = aMessage.json;
+    let browser = aMessage.target;
+    switch (aMessage.name) {
+      case "Content:ZoomToRect": {
+        let { presShellId, viewId } = json;
+        let rect = Rect.fromRect(json.rect);
+        if (this.isRectZoomedIn(rect, browser.contentViewportBounds)) {
+          // If we're already zoomed in, zoom out instead.
+          rect = new Rect(0,0,0,0);
+        }
+        let data = [rect.x, rect.y, rect.width, rect.height, presShellId, viewId].join(",");
+        Services.obs.notifyObservers(null, "apzc-zoom-to-rect", data);
+      }
+    }
+  },
+
+  /**
+   * Check to see if the area of the rect visible in the viewport is
+   * approximately the max area of the rect we can show.
+   * Based on code from BrowserElementPanning.js
+   */
+  isRectZoomedIn: function (aRect, aViewport) {
+    let overlap = aViewport.intersect(aRect);
+    let overlapArea = overlap.width * overlap.height;
+    let availHeight = Math.min(aRect.width * aViewport.height / aViewport.width, aRect.height);
+    let showing = overlapArea / (aRect.width * availHeight);
+    let ratioW = (aRect.width / aViewport.width);
+    let ratioH = (aRect.height / aViewport.height);
+
+    return (showing > 0.9 && (ratioW > 0.9 || ratioH > 0.9));
+  },
+
+  _resetDisplayPort: function () {
+    // Start off with something reasonable. The apzc will handle these
+    // calculations once scrolling starts.
+    let doc = Browser.selectedBrowser.contentDocument.documentElement;
+    // While running tests, sometimes this can be null. If we don't have a
+    // root document, there's no point in setting a scrollable display port.
+    if (!doc) {
+      return;
+    }
+    let win = Browser.selectedBrowser.contentWindow;
+    let factor = 0.2;
+    let portX = 0;
+    let portY = 0;
+    let portWidth = ContentAreaObserver.width;
+    let portHeight = ContentAreaObserver.height;
+
+    if (portWidth < doc.scrollWidth) {
+      portWidth += ContentAreaObserver.width * factor;
+      if (portWidth > doc.scrollWidth) {
+        portWidth = doc.scrollWidth;
+      }
+    }
+    if (portHeight < doc.scrollHeight) {
+      portHeight += ContentAreaObserver.height * factor;
+      if (portHeight > doc.scrollHeight) {
+        portHeight = doc.scrollHeight;
+      }
+    }
+    if (win.scrollX > 0) {
+      portX -= ContentAreaObserver.width * factor;
+    }
+    if (win.scrollY > 0) {
+      portY -= ContentAreaObserver.height * factor;
+    }
+    let cwu = Browser.selectedBrowser.contentWindow
+                      .QueryInterface(Ci.nsIInterfaceRequestor)
+                      .getInterface(Ci.nsIDOMWindowUtils);
+    cwu.setDisplayPortForElement(portX, portY,
+                                 portWidth, portHeight,
+                                 Browser.selectedBrowser.contentDocument.documentElement,
+                                 0);
+  }
+};