Avoid div-by-zero in thumb quad computation

We used to have trouble when the max scroll offset was zero. With this
cl we detect this case and set a reasonable scroll thumb offset.

BUG=None

Review URL: https://codereview.chromium.org/409193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284773 0039d316-1c4b-4281-b951-d872f2087c98

cc/layers/scrollbar_layer_impl_base.cc

@@ -212,9 +212,13 @@ gfx::Rect ScrollbarLayerImplBase::ComputeThumbQuadRect() const {
   // With the length known, we can compute the thumb's position.
   float clamped_current_pos =
       std::min(std::max(current_pos_, 0.f), static_cast<float>(maximum_));
-  float ratio = clamped_current_pos / maximum_;
-  float max_offset = track_length - thumb_length;
-  int thumb_offset = static_cast<int>(ratio * max_offset) + TrackStart();
+
+  int thumb_offset = TrackStart();
+  if (maximum_ > 0) {
+    float ratio = clamped_current_pos / maximum_;
+    float max_offset = track_length - thumb_length;
+    thumb_offset += static_cast<int>(ratio * max_offset);
+  }
 
   float thumb_thickness_adjustment =
       thumb_thickness * (1.f - thumb_thickness_scale_factor_);

cc/layers/scrollbar_layer_unittest.cc

@@ -274,7 +274,7 @@ TEST(ScrollbarLayerTest, ThumbRect) {
 
 TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
   const int kThumbThickness = 3;
-  const int kTrackStart = 0;
+  const int kTrackStart = 1;
   const int kTrackLength = 100;
 
   LayerTreeSettings layer_tree_settings;
@@ -303,7 +303,7 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     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.
@@ -319,7 +319,7 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     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;
@@ -337,7 +337,23 @@ TEST(ScrollbarLayerTest, SolidColorDrawQuads) {
     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);
   }
 }