diff -r 000000000000 -r 6474c204b198 widget/cocoa/nsScreenManagerCocoa.mm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/widget/cocoa/nsScreenManagerCocoa.mm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,123 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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/. */
+
+#include "nsScreenManagerCocoa.h"
+#include "nsObjCExceptions.h"
+#include "nsCOMPtr.h"
+#include "nsCocoaUtils.h"
+
+NS_IMPL_ISUPPORTS(nsScreenManagerCocoa, nsIScreenManager)
+
+nsScreenManagerCocoa::nsScreenManagerCocoa()
+{
+}
+
+nsScreenManagerCocoa::~nsScreenManagerCocoa()
+{
+}
+
+nsScreenCocoa*
+nsScreenManagerCocoa::ScreenForCocoaScreen(NSScreen *screen)
+{
+    for (uint32_t i = 0; i < mScreenList.Length(); ++i) {
+        nsScreenCocoa* sc = mScreenList[i];
+        if (sc->CocoaScreen() == screen) {
+            // doesn't addref
+            return sc;
+        }
+    }
+
+    // didn't find it; create and insert
+    nsRefPtr<nsScreenCocoa> sc = new nsScreenCocoa(screen);
+    mScreenList.AppendElement(sc);
+    return sc.get();
+}
+
+NS_IMETHODIMP
+nsScreenManagerCocoa::ScreenForRect (int32_t aX, int32_t aY, int32_t aWidth, int32_t aHeight,
+                                     nsIScreen **outScreen)
+{
+    NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+    NSEnumerator *screenEnum = [[NSScreen screens] objectEnumerator];
+    NSRect inRect = nsCocoaUtils::GeckoRectToCocoaRect(nsIntRect(aX, aY, aWidth, aHeight));
+    NSScreen *screenWindowIsOn = [NSScreen mainScreen];
+    float greatestArea = 0;
+
+    while (NSScreen *screen = [screenEnum nextObject]) {
+        NSDictionary *desc = [screen deviceDescription];
+        if ([desc objectForKey:NSDeviceIsScreen] == nil)
+            continue;
+
+        NSRect r = NSIntersectionRect([screen frame], inRect);
+        float area = r.size.width * r.size.height;
+        if (area > greatestArea) {
+            greatestArea = area;
+            screenWindowIsOn = screen;
+        }
+    }
+
+    *outScreen = ScreenForCocoaScreen(screenWindowIsOn);
+    NS_ADDREF(*outScreen);
+    return NS_OK;
+
+    NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}
+
+NS_IMETHODIMP
+nsScreenManagerCocoa::GetPrimaryScreen (nsIScreen **outScreen)
+{
+    NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+    // the mainScreen is the screen with the "key window" (focus, I assume?)
+    NSScreen *sc = [[NSScreen screens] objectAtIndex:0];
+
+    *outScreen = ScreenForCocoaScreen(sc);
+    NS_ADDREF(*outScreen);
+
+    return NS_OK;
+
+    NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}
+
+NS_IMETHODIMP
+nsScreenManagerCocoa::GetNumberOfScreens (uint32_t *aNumberOfScreens)
+{
+    NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+    NSArray *ss = [NSScreen screens];
+
+    *aNumberOfScreens = [ss count];
+
+    return NS_OK;
+
+    NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}
+
+NS_IMETHODIMP
+nsScreenManagerCocoa::GetSystemDefaultScale(float *aDefaultScale)
+{
+    *aDefaultScale = 1.0f;
+    return NS_OK;
+}
+
+NS_IMETHODIMP
+nsScreenManagerCocoa::ScreenForNativeWidget (void *nativeWidget, nsIScreen **outScreen)
+{
+    NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+    NSWindow *window = static_cast<NSWindow*>(nativeWidget);
+    if (window) {
+        nsIScreen *screen = ScreenForCocoaScreen([window screen]);
+        *outScreen = screen;
+        NS_ADDREF(*outScreen);
+        return NS_OK;
+    }
+
+    *outScreen = nullptr;
+    return NS_OK;
+
+    NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}