diff --git a/gfx/thebes/public/gfxQtPlatform.h b/gfx/thebes/public/gfxQtPlatform.h
index 1a76a858db7..9c0b4a8c37e 100644
--- a/gfx/thebes/public/gfxQtPlatform.h
+++ b/gfx/thebes/public/gfxQtPlatform.h
@@ -135,6 +135,7 @@ public:
 
 protected:
     static gfxFontconfigUtils *sFontconfigUtils;
+    void InitDisplayCaps();
 
 private:
     virtual qcms_profile *GetPlatformCMSOutputProfile();
diff --git a/gfx/thebes/src/gfxQtPlatform.cpp b/gfx/thebes/src/gfxQtPlatform.cpp
index 05620e1be16..bce3aafc251 100644
--- a/gfx/thebes/src/gfxQtPlatform.cpp
+++ b/gfx/thebes/src/gfxQtPlatform.cpp
@@ -39,7 +39,7 @@
 #include <QPixmap>
 #include <QX11Info>
 #include <QApplication>
-#include <QWidget>
+#include <QDesktopWidget>
 
 #include "gfxQtPlatform.h"
 
@@ -583,3 +583,12 @@ gfxQtPlatform::SetPrefFontEntries(const nsCString& aKey, nsTArray<nsRefPtr<gfxFo
 }
 #endif
 
+void
+gfxQtPlatform::InitDisplayCaps()
+{
+    QDesktopWidget* rootWindow = qApp->desktop();
+    sDPI = rootWindow->logicalDpiY(); // y-axis DPI for fonts
+    if (sDPI <= 0)
+        sDPI = 96; // something more sensible
+}
+