diff -r 000000000000 -r 6474c204b198 widget/qt/nsScreenManagerQt.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/widget/qt/nsScreenManagerQt.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,117 @@
+/* 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 <QGuiApplication>
+#include <QScreen>
+
+#include "nsScreenManagerQt.h"
+#include "nsScreenQt.h"
+
+nsScreenManagerQt::nsScreenManagerQt()
+{
+    mInitialized = false;
+    desktop = 0;
+    screens = 0;
+}
+
+nsScreenManagerQt::~nsScreenManagerQt()
+{
+    delete [] screens;
+}
+
+// addref, release, QI
+NS_IMPL_ISUPPORTS(nsScreenManagerQt, nsIScreenManager)
+
+void nsScreenManagerQt::init()
+{
+    if (mInitialized)
+        return;
+
+    nScreens = QGuiApplication::screens().size();
+    screens = new nsCOMPtr<nsIScreen>[nScreens];
+
+    for (int i = 0; i < nScreens; ++i)
+        screens[i] = new nsScreenQt(i);
+    mInitialized = true;
+}
+
+//
+// ScreenForRect
+//
+// Returns the screen that contains the rectangle. If the rect overlaps
+// multiple screens, it picks the screen with the greatest area of intersection.
+//
+// The coordinates are in pixels (not twips) and in screen coordinates.
+//
+NS_IMETHODIMP
+nsScreenManagerQt::ScreenForRect(int32_t inLeft, int32_t inTop,
+                                 int32_t inWidth, int32_t inHeight,
+                                 nsIScreen **outScreen)
+{
+    if (!mInitialized)
+        init();
+
+    QRect r(inLeft, inTop, inWidth, inHeight);
+    int best = 0;
+    int area = 0;
+    for (int i = 0; i < nScreens; ++i) {
+        const QRect& rect = QGuiApplication::screens()[i]->geometry();
+        QRect intersection = r&rect;
+        int a = intersection.width()*intersection.height();
+        if (a > area) {
+            best = i;
+            area = a;
+        }
+    }
+
+    NS_IF_ADDREF(*outScreen = screens[best]);
+    return NS_OK;
+}
+
+//
+// GetPrimaryScreen
+//
+// The screen with the menubar/taskbar. This shouldn't be needed very
+// often.
+//
+NS_IMETHODIMP
+nsScreenManagerQt::GetPrimaryScreen(nsIScreen **aPrimaryScreen)
+{
+    if (!desktop)
+        init();
+
+    NS_IF_ADDREF(*aPrimaryScreen = screens[0]);
+    return NS_OK;
+}
+
+//
+// GetNumberOfScreens
+//
+// Returns how many physical screens are available.
+//
+NS_IMETHODIMP
+nsScreenManagerQt::GetNumberOfScreens(uint32_t *aNumberOfScreens)
+{
+    if (!desktop)
+        init();
+
+    *aNumberOfScreens = nScreens;
+    return NS_OK;
+}
+
+NS_IMETHODIMP
+nsScreenManagerQt::GetSystemDefaultScale(float *aDefaultScale)
+{
+    *aDefaultScale = 1.0f;
+    return NS_OK;
+}
+
+NS_IMETHODIMP
+nsScreenManagerQt::ScreenForNativeWidget(void *aWidget, nsIScreen **outScreen)
+{
+    // I don't know how to go from GtkWindow to nsIScreen, especially
+    // given xinerama and stuff, so let's just do this
+    QRect rect(0, 0, 1, 1);
+    return ScreenForRect(rect.x(), rect.y(), rect.width(), rect.height(), outScreen);
+}