Backout bug 426788

gfx/src/thebes/nsThebesDeviceContext.cpp

@@ -685,11 +685,16 @@ nsThebesDeviceContext::SetDPI()
 
     if (dotsArePixels) {
         if (prefDevPixelsPerCSSPixel <= 0) {
-            // Round down to multiple of 96, which is the number of dev pixels
-            // per CSS pixel.  Then, divide that into AppUnitsPerCSSPixel()
-            // to get the number of app units per dev pixel.  The PR_MAXes are
-            // to make sure we don't end up dividing by zero.
-            PRUint32 roundedDPIScaleFactor = dpi/96;
+            // First figure out the closest multiple of 96, which is the number of
+            // dev pixels per CSS pixel.  Then, divide that into AppUnitsPerCSSPixel()
+            // to get the number of app units per dev pixel.  The PR_MAXes are to
+            // make sure we don't end up dividing by zero.
+            PRUint32 roundedDPIScaleFactor = (dpi + 48)/96;
+#ifdef MOZ_WIDGET_GTK2
+            // be more conservative about activating scaling on GTK2, since the dpi
+            // information is more likely to be wrong
+            roundedDPIScaleFactor = dpi/96;
+#endif
             mAppUnitsPerDevNotScaledPixel =
                 PR_MAX(1, AppUnitsPerCSSPixel() / PR_MAX(1, roundedDPIScaleFactor));
         } else {