cc: Don't discard framebuffer when using partial swaps

cc/output/direct_renderer.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/direct_renderer.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/containers/hash_tables.h"
@@ skipping 315 matching lines @@
 void DirectRenderer::DrawRenderPass(DrawingFrame* frame,
                                     const RenderPass* render_pass,
                                     bool allow_partial_swap) {
   TRACE_EVENT0("cc", "DirectRenderer::DrawRenderPass");
   if (!UseRenderPass(frame, render_pass))
     return;
 
   bool using_scissor_as_optimization =
       Capabilities().using_partial_swap && allow_partial_swap;
   gfx::RectF render_pass_scissor;
+  bool draw_rect_covers_full_surface = true;
+  if (frame->current_render_pass == frame->root_render_pass &&
+      !client_->DeviceViewport().Contains(
+           gfx::Rect(output_surface_->SurfaceSize())))
+    draw_rect_covers_full_surface = false;
 
   if (using_scissor_as_optimization) {
     render_pass_scissor = ComputeScissorRectForRenderPass(frame);
     SetScissorTestRectInDrawSpace(frame, render_pass_scissor);
+    if (!render_pass_scissor.Contains(frame->current_render_pass->output_rect))
+      draw_rect_covers_full_surface = false;
   }
 
   if (frame->current_render_pass != frame->root_render_pass ||
       settings_->should_clear_root_render_pass) {
-    if (NeedDeviceClip(frame))
+    if (NeedDeviceClip(frame)) {
       SetScissorTestRect(DeviceClipRect(frame));
-    else if (!using_scissor_as_optimization)
+      draw_rect_covers_full_surface = false;
+    } else if (!using_scissor_as_optimization) {
       EnsureScissorTestDisabled();
-    ClearFramebuffer(frame);
+    }
+
+    bool has_external_stencil_test =
+        output_surface_->HasExternalStencilTest() &&
+        frame->current_render_pass == frame->root_render_pass;
+
+    DiscardPixels(has_external_stencil_test, draw_rect_covers_full_surface);
+    ClearFramebuffer(frame, has_external_stencil_test);
   }
 
   const QuadList& quad_list = render_pass->quad_list;
   for (QuadList::ConstBackToFrontIterator it = quad_list.BackToFrontBegin();
        it != quad_list.BackToFrontEnd();
        ++it) {
     const DrawQuad& quad = *(*it);
     bool should_skip_quad = false;
 
     if (using_scissor_as_optimization) {
@@ skipping 61 matching lines @@
   return render_pass->output_rect.size();
 }
 
 // static
 ResourceFormat DirectRenderer::RenderPassTextureFormat(
     const RenderPass* render_pass) {
   return RGBA_8888;
 }
 
 }  // namespace cc