Avoid div-by-zero in thumb quad computation

cc/layers/scrollbar_layer_unittest.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 "base/containers/hash_tables.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/layers/append_quads_data.h"
 #include "cc/layers/painted_scrollbar_layer.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
 #include "cc/layers/scrollbar_layer_interface.h"
@@ skipping 256 matching lines @@
   // position).
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(30, 12, 50, 6));
 
   UPDATE_AND_EXTRACT_LAYER_POINTERS();
   EXPECT_EQ(gfx::Rect(44, 0, 6, 4).ToString(),
             scrollbar_layer_impl->ComputeThumbQuadRect().ToString());
 }
 
 TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
   const int kThumbThickness = 3;
-  const int kTrackStart = 0;
+  const int kTrackStart = 1;

[weiliangc, 2014/07/22 18:49:46]

I don't understand why we need to change TrackStart constant in test?

[Ian Vollick, 2014/07/22 18:55:00]

It's a good question. I did this so that I can more reliably check that we set
the TrackStart value when we avoid the if block in scrollbar_layer_base_impl
when maximum_ is zero (rather than coincidentally winding up with zero if the
numerics fell apart).

I also think it's a nicer value than zero, even for the other tests -- if it's
non-zero, we can actually see it being used.

[weiliangc, 2014/07/22 19:00:12]

Ah didn't realize the first test case has 40->39 and second test case 80->78 is
because it's 2x the first time, thought second case decremented more than first
test case because of rounding issue.

Yeah, looks good. 

   const int kTrackLength = 100;
 
   LayerTreeSettings layer_tree_settings;
   scoped_ptr<FakeLayerTreeHost> host =
       FakeLayerTreeHost::Create(layer_tree_settings);
 
   scoped_ptr<Scrollbar> scrollbar(new FakeScrollbar(false, true, true));
   LayerImpl* layer_impl_tree_root = LayerImplForScrollAreaAndScrollbar(
       host.get(), scrollbar.Pass(), false, true, kThumbThickness, kTrackStart);
   ScrollbarLayerImplBase* scrollbar_layer_impl =
       static_cast<SolidColorScrollbarLayerImpl*>(
           layer_impl_tree_root->children()[1]);
   scrollbar_layer_impl->SetBounds(gfx::Size(kTrackLength, kThumbThickness));
   scrollbar_layer_impl->SetCurrentPos(10.f);
   scrollbar_layer_impl->SetMaximum(100);
   scrollbar_layer_impl->SetVisibleToTotalLengthRatio(0.4f);
 
   // Thickness should be overridden to 3.
   {
     MockOcclusionTracker<LayerImpl> occlusion_tracker;
     scoped_ptr<RenderPass> render_pass = RenderPass::Create();
     AppendQuadsData data;
     scrollbar_layer_impl->AppendQuads(
         render_pass.get(), occlusion_tracker, &data);
 
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(6, 0, 40, 3), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(6, 0, 39, 3), quads[0]->rect);
   }
 
   // Contents scale should scale the draw quad.
   scrollbar_layer_impl->draw_properties().contents_scale_x = 2.f;
   scrollbar_layer_impl->draw_properties().contents_scale_y = 2.f;
   {
     MockOcclusionTracker<LayerImpl> occlusion_tracker;
     scoped_ptr<RenderPass> render_pass = RenderPass::Create();
     AppendQuadsData data;
     scrollbar_layer_impl->AppendQuads(
         render_pass.get(), occlusion_tracker, &data);
 
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(12, 0, 80, 6), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(12, 0, 78, 6), quads[0]->rect);
   }
   scrollbar_layer_impl->draw_properties().contents_scale_x = 1.f;
   scrollbar_layer_impl->draw_properties().contents_scale_y = 1.f;
 
   // For solid color scrollbars, position and size should reflect the
   // current viewport state.
   scrollbar_layer_impl->SetVisibleToTotalLengthRatio(0.2f);
   {
     MockOcclusionTracker<LayerImpl> occlusion_tracker;
     scoped_ptr<RenderPass> render_pass = RenderPass::Create();
     AppendQuadsData data;
     scrollbar_layer_impl->AppendQuads(
         render_pass.get(), occlusion_tracker, &data);
 
     const QuadList& quads = render_pass->quad_list;
     ASSERT_EQ(1u, quads.size());
     EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
-    EXPECT_RECT_EQ(gfx::Rect(8, 0, 20, 3), quads[0]->rect);
+    EXPECT_RECT_EQ(gfx::Rect(8, 0, 19, 3), quads[0]->rect);
+  }
+
+  // We shouldn't attempt div-by-zero when the maximum is zero.
+  scrollbar_layer_impl->SetCurrentPos(0.f);
+  scrollbar_layer_impl->SetMaximum(0);
+  {
+    MockOcclusionTracker<LayerImpl> occlusion_tracker;
+    scoped_ptr<RenderPass> render_pass = RenderPass::Create();
+    AppendQuadsData data;
+    scrollbar_layer_impl->AppendQuads(
+        render_pass.get(), occlusion_tracker, &data);
+
+    const QuadList& quads = render_pass->quad_list;
+    ASSERT_EQ(1u, quads.size());
+    EXPECT_EQ(DrawQuad::SOLID_COLOR, quads[0]->material);
+    EXPECT_RECT_EQ(gfx::Rect(1, 0, 19, 3), quads[0]->rect);
   }
 }
 
 TEST(ScrollbarLayerTest, LayerDrivenSolidColorDrawQuads) {
   const int kThumbThickness = 3;
   const int kTrackStart = 0;
   const int kTrackLength = 10;
 
   LayerTreeSettings layer_tree_settings;
   scoped_ptr<FakeLayerTreeHost> host =
@@ skipping 542 matching lines @@
   TestScale(gfx::Rect(1240, 0, 15, 1333), 2.7754839f);
   TestScale(gfx::Rect(1240, 0, 15, 677), 2.46677136f);
 
   // Horizontal Scrollbars.
   TestScale(gfx::Rect(0, 1240, 1333, 15), 2.7754839f);
   TestScale(gfx::Rect(0, 1240, 677, 15), 2.46677136f);
 }
 
 }  // namespace
 }  // namespace cc