diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index 0dc431ae6b5..e3fd99f9db7 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -1577,7 +1577,15 @@ GetBackgroundClip(gfxContext *aCtx, PRUint8 aBackgroundClip,
aClipState->mClippedRadii = aBGRadii;
if (aBackgroundClip != NS_STYLE_BG_CLIP_BORDER) {
nsMargin border = aForFrame->GetUsedBorder();
- if (aBackgroundClip != NS_STYLE_BG_CLIP_PADDING) {
+ if (aBackgroundClip == NS_STYLE_BG_CLIP_MOZ_ALMOST_PADDING) {
+ // Reduce |border| by 1px (device pixels) on all sides, if
+ // possible, so that we don't get antialiasing seams between the
+ // background and border.
+ border.top = NS_MAX(0, border.top - aAppUnitsPerPixel);
+ border.right = NS_MAX(0, border.right - aAppUnitsPerPixel);
+ border.bottom = NS_MAX(0, border.bottom - aAppUnitsPerPixel);
+ border.left = NS_MAX(0, border.left - aAppUnitsPerPixel);
+ } else if (aBackgroundClip != NS_STYLE_BG_CLIP_PADDING) {
NS_ASSERTION(aBackgroundClip == NS_STYLE_BG_CLIP_CONTENT,
"unexpected background-clip");
border += aForFrame->GetUsedPadding();
@@ -2381,8 +2389,13 @@ nsCSSRendering::PaintBackgroundWithSC(nsPresContext* aPresContext,
currentBackgroundClip = bg->BottomLayer().mClip;
isSolidBorder =
(aFlags & PAINTBG_WILL_PAINT_BORDER) && IsOpaqueBorder(aBorder);
- if (isSolidBorder && currentBackgroundClip == NS_STYLE_BG_CLIP_BORDER)
- currentBackgroundClip = NS_STYLE_BG_CLIP_PADDING;
+ if (isSolidBorder && currentBackgroundClip == NS_STYLE_BG_CLIP_BORDER) {
+ // If we have rounded corners, we need to inflate the background
+ // drawing area a bit to avoid seams between the border and
+ // background.
+ currentBackgroundClip = haveRoundedCorners ?
+ NS_STYLE_BG_CLIP_MOZ_ALMOST_PADDING : NS_STYLE_BG_CLIP_PADDING;
+ }
GetBackgroundClip(ctx, currentBackgroundClip, aForFrame, aBorderArea,
aDirtyRect, haveRoundedCorners, bgRadii, appUnitsPerPixel,
@@ -2428,8 +2441,10 @@ nsCSSRendering::PaintBackgroundWithSC(nsPresContext* aPresContext,
const nsStyleBackground::Layer &layer = bg->mLayers[i];
if (!aBGClipRect) {
PRUint8 newBackgroundClip = layer.mClip;
- if (isSolidBorder && newBackgroundClip == NS_STYLE_BG_CLIP_BORDER)
- newBackgroundClip = NS_STYLE_BG_CLIP_PADDING;
+ if (isSolidBorder && newBackgroundClip == NS_STYLE_BG_CLIP_BORDER) {
+ newBackgroundClip = haveRoundedCorners ?
+ NS_STYLE_BG_CLIP_MOZ_ALMOST_PADDING : NS_STYLE_BG_CLIP_PADDING;
+ }
if (currentBackgroundClip != newBackgroundClip || !clipSet) {
currentBackgroundClip = newBackgroundClip;
// If clipSet is false that means this is the bottom layer and we
diff --git a/layout/base/nsStyleConsts.h b/layout/base/nsStyleConsts.h
index 7b0cec3c219..9addf7f458e 100644
--- a/layout/base/nsStyleConsts.h
+++ b/layout/base/nsStyleConsts.h
@@ -273,6 +273,12 @@ static inline mozilla::css::Side operator++(mozilla::css::Side& side, int) {
#define NS_STYLE_BG_CLIP_BORDER 0
#define NS_STYLE_BG_CLIP_PADDING 1
#define NS_STYLE_BG_CLIP_CONTENT 2
+// A magic value that we use for our "pretend that background-clip is
+// 'padding' when we have a solid border" optimization. This isn't
+// actually equal to NS_STYLE_BG_CLIP_PADDING because using that
+// causes antialiasing seams between the background and border. This
+// is a backend-only value.
+#define NS_STYLE_BG_CLIP_MOZ_ALMOST_PADDING 127
// See nsStyleBackground
#define NS_STYLE_BG_INLINE_POLICY_EACH_BOX 0
diff --git a/layout/reftests/border-radius/curved-border-background-nogap-ref.html b/layout/reftests/border-radius/curved-border-background-nogap-ref.html
new file mode 100644
index 00000000000..67aead2bcab
--- /dev/null
+++ b/layout/reftests/border-radius/curved-border-background-nogap-ref.html
@@ -0,0 +1,9 @@
+
+
+
diff --git a/layout/reftests/border-radius/curved-border-background-nogap.html b/layout/reftests/border-radius/curved-border-background-nogap.html
new file mode 100644
index 00000000000..01207e7f044
--- /dev/null
+++ b/layout/reftests/border-radius/curved-border-background-nogap.html
@@ -0,0 +1,36 @@
+
+
+
diff --git a/layout/reftests/border-radius/reftest.list b/layout/reftests/border-radius/reftest.list
index 2a244d4e0d3..656eb46cca0 100644
--- a/layout/reftests/border-radius/reftest.list
+++ b/layout/reftests/border-radius/reftest.list
@@ -79,3 +79,6 @@ random-if(winWidget) HTTP(..) == corner-joins-2.xhtml corner-joins-2-ref.xhtml
== zero-radius-clip-1.html zero-radius-clip-ref.html
== iframe-1.html iframe-1-ref.html
+
+# Test for antialiasing gaps between background and border
+fails-if(winWidget) == curved-border-background-nogap.html curved-border-background-nogap-ref.html
diff --git a/layout/reftests/box-shadow/boxshadow-mixed-ref.html b/layout/reftests/box-shadow/boxshadow-mixed-ref.html
index 542872343c4..3c71b12e200 100644
--- a/layout/reftests/box-shadow/boxshadow-mixed-ref.html
+++ b/layout/reftests/box-shadow/boxshadow-mixed-ref.html
@@ -1 +1 @@
-inset and outset
+inset and outset