Ensure square list bullets are always square. (Bug 376690) r=roc

layout/generic/nsBulletFrame.cpp

@@ -273,9 +273,25 @@ nsBulletFrame::PaintBullet(nsIRenderingContext& aRenderingContext, nsPoint aPt,
     break;
 
   case NS_STYLE_LIST_STYLE_SQUARE:
-    aRenderingContext.FillRect(mPadding.left + aPt.x, mPadding.top + aPt.y,
-                               mRect.width - (mPadding.left + mPadding.right),
-                               mRect.height - (mPadding.top + mPadding.bottom));
+    {
+      nsRect rect(mPadding.TopLeft() + aPt,
+                  nsSize(mRect.width - mPadding.LeftRight(),
+                         mRect.height - mPadding.TopBottom()));
+      // Snap the height and the width of the rectangle to device pixels,
+      // and then center the result within the original rectangle, so that
+      // all square bullets at the same font size have the same visual
+      // size (bug 376690).
+      // FIXME: We should really only do this if we're not transformed
+      // (like gfxContext::UserToDevicePixelSnapped does).
+      nsPresContext *pc = PresContext();
+      nsRect snapRect(rect.x, rect.y, 
+                      pc->RoundAppUnitsToNearestDevPixels(rect.width),
+                      pc->RoundAppUnitsToNearestDevPixels(rect.height));
+      snapRect.MoveBy((rect.width - snapRect.width) / 2,
+                      (rect.height - snapRect.height) / 2);
+      aRenderingContext.FillRect(snapRect.x, snapRect.y,
+                                 snapRect.width, snapRect.height);
+    }
     break;
 
   case NS_STYLE_LIST_STYLE_DECIMAL: