Bug 1148418 - Make nsDisplayBackgroundColor::Paint use a higher precision floating point type to avoid scrolling artifacts. r=mattwoodrow

layout/base/nsDisplayList.cpp

@@ -2938,11 +2938,28 @@ nsDisplayBackgroundColor::Paint(nsDisplayListBuilder* aBuilder,
 
   nsRect borderBox = nsRect(ToReferenceFrame(), mFrame->GetSize());
 
+#if 0
+  // See https://bugzilla.mozilla.org/show_bug.cgi?id=1148418#c21 for why this
+  // results in a precision induced rounding issue that makes the rect one
+  // pixel shorter in rare cases. Disabled in favor of the old code for now.
+  // Note that the pref layout.css.devPixelsPerPx needs to be set to 1 to
+  // reproduce the bug.
   Rect rect = NSRectToSnappedRect(borderBox,
                                   mFrame->PresContext()->AppUnitsPerDevPixel(),
                                   aDrawTarget);
   ColorPattern color(ToDeviceColor(mColor));
   aDrawTarget.FillRect(rect, color);
+#else
+  gfxContext* ctx = aCtx->ThebesContext();
+
+  gfxRect bounds =
+    nsLayoutUtils::RectToGfxRect(borderBox, mFrame->PresContext()->AppUnitsPerDevPixel());
+
+  ctx->SetColor(mColor);
+  ctx->NewPath();
+  ctx->Rectangle(bounds, true);
+  ctx->Fill();
+#endif
 }
 
 nsRegion