From 257cfd9290f1b235a7b3e46db35b2e58fcf0ad00 Mon Sep 17 00:00:00 2001
From: Cameron Kaiser <classilla@floodgap.com>
Date: Tue, 18 Jul 2017 20:55:41 -0700
Subject: [PATCH] #416: M1265568 M1305036

---
 gfx/thebes/gfxPlatform.h        |  6 ++++++
 gfx/thebes/gfxPlatformMac.h     |  5 +++++
 layout/generic/TextOverflow.cpp | 19 +++++++++++++++++++
 layout/generic/nsTextFrame.cpp  | 12 ++++++++++++
 4 files changed, 42 insertions(+)

diff --git a/gfx/thebes/gfxPlatform.h b/gfx/thebes/gfxPlatform.h
index e4325e46b..0ce966519 100644
--- a/gfx/thebes/gfxPlatform.h
+++ b/gfx/thebes/gfxPlatform.h
@@ -403,6 +403,12 @@ public:
      */
     virtual bool RequiresLinearZoom() { return false; }
 
+    /**
+     * Whether the frame->StyleFont().mFont.smoothing field is respected by
+     * text rendering on this platform.
+     */
+    virtual bool RespectsFontStyleSmoothing() const { return false; }
+
     /**
      * Whether to check all font cmaps during system font fallback
      */
diff --git a/gfx/thebes/gfxPlatformMac.h b/gfx/thebes/gfxPlatformMac.h
index 089186ce7..4e6e5426d 100644
--- a/gfx/thebes/gfxPlatformMac.h
+++ b/gfx/thebes/gfxPlatformMac.h
@@ -100,6 +100,11 @@ public:
       return true;
     }
 
+    bool RespectsFontStyleSmoothing() const override {
+      // gfxMacFont respects the font smoothing hint.
+      return true;
+    }
+
     bool RequiresAcceleratedGLContextForCompositorOGL() const override {
       // On OS X in a VM, unaccelerated CompositorOGL shows black flashes, so we
       // require accelerated GL for CompositorOGL but allow unaccelerated GL for
diff --git a/layout/generic/TextOverflow.cpp b/layout/generic/TextOverflow.cpp
index 8111c6d78..5a1db5929 100644
--- a/layout/generic/TextOverflow.cpp
+++ b/layout/generic/TextOverflow.cpp
@@ -180,6 +180,25 @@ public:
       nsLayoutUtils::GetTextShadowRectsUnion(mRect, mFrame);
     return mRect.Union(shadowRect);
   }
+
+  virtual nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) override
+  {
+#ifdef XP_MACOSX
+    {
+#else
+    if (gfxPlatform::GetPlatform()->RespectsFontStyleSmoothing()) {
+#endif
+      // On OS X, web authors can turn off subpixel text rendering using the
+      // CSS property -moz-osx-font-smoothing. If they do that, we don't need
+      // to use component alpha layers for the affected text.
+      if (mFrame->StyleFont()->mFont.smoothing == NS_FONT_SMOOTHING_GRAYSCALE) {
+        return nsRect();
+      }
+    }
+    bool snap;
+    return GetBounds(aBuilder, &snap);
+  }
+
   virtual void Paint(nsDisplayListBuilder* aBuilder,
                      nsRenderingContext* aCtx) override;
 
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index 349866f35..819ffd1ee 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -4680,6 +4680,18 @@ public:
 
   virtual nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) override
   {
+#ifdef XP_MACOSX
+    {
+#else
+    if (gfxPlatform::GetPlatform()->RespectsFontStyleSmoothing()) {
+#endif
+      // On OS X, web authors can turn off subpixel text rendering using the
+      // CSS property -moz-osx-font-smoothing. If they do that, we don't need
+      // to use component alpha layers for the affected text.
+      if (mFrame->StyleFont()->mFont.smoothing == NS_FONT_SMOOTHING_GRAYSCALE) {
+        return nsRect();
+      }
+    }
     bool snap;
     return GetBounds(aBuilder, &snap);
   }