Use more than double buffering in SoftwareOutputDeviceMac

content/browser/compositor/software_output_device_mac.h

Index: content/browser/compositor/software_output_device_mac.h
diff --git a/content/browser/compositor/software_output_device_mac.h b/content/browser/compositor/software_output_device_mac.h
index 77c61fcd7e5ff5bf3e81251a25b4c123d453514c..8548d16d3c6b353d59e06a7b22741b5a8235c883 100644
--- a/content/browser/compositor/software_output_device_mac.h
+++ b/content/browser/compositor/software_output_device_mac.h
@@ -42,27 +42,35 @@ class SoftwareOutputDeviceMac :
       const gfx::VSyncProvider::UpdateVSyncCallback& callback) override;
 
  private:
-  bool EnsureBuffersExist();
-
-  // Copy the pixels from the previous buffer to the new buffer.
-  void CopyPreviousBufferDamage(const SkRegion& new_damage_rect);
-
-  ui::Compositor* compositor_;
+  //
+  struct Buffer {
+    Buffer();
+    ~Buffer();
+    base::ScopedCFTypeRef<IOSurfaceRef> io_surface;
+    // The damage of all BeginPaints since this buffer was the back buffer.
+    SkRegion accumulated_damage;
+  };
+
+  // Copy the pixels from the previous buffer to the new buffer, and union
+  // |new_damage_rect| into all |buffer_queue_|'s accumulated damages.
+  void UpdateAndCopyBufferDamage(Buffer* previous_paint_buffer,
+                                 const SkRegion& new_damage_rect);
+
+  ui::Compositor* compositor_ = nullptr;
   gfx::Size pixel_size_;
-  float scale_factor_;
+  float scale_factor_ = 1;
 
   // This surface is double-buffered. The two buffers are in |io_surfaces_|,

[jbauman, 2017/05/05 19:19:25]

Fix comment.

[ccameron, 2017/05/05 22:06:08]

Done.

   // and the index of the current buffer is |current_buffer_|.
-  base::ScopedCFTypeRef<IOSurfaceRef> io_surfaces_[2];
-  int current_index_;
+  std::list<std::unique_ptr<Buffer>> buffer_queue_;
 
-  // The previous frame's damage rectangle. Used to copy unchanged content
-  // between buffers in CopyPreviousBufferDamage.
-  SkRegion previous_buffer_damage_region_;
+  // A pointer to the last element of |buffer_queue_| during paint. It is only
+  // valid between BeginPaint and EndPaint.
+  Buffer* current_paint_buffer_ = nullptr;
 
-  // The SkCanvas wrapps the mapped current IOSurface. It is valid only between
-  // BeginPaint and EndPaint.
-  std::unique_ptr<SkCanvas> canvas_;
+  // The SkCanvas wraps the mapped |current_paint_buffer_|'s IOSurface. It is
+  // valid only between BeginPaint and EndPaint.
+  std::unique_ptr<SkCanvas> current_paint_canvas_ = nullptr;
 
   gfx::VSyncProvider::UpdateVSyncCallback update_vsync_callback_;