Don't let scrollbar thumbs run off their tracks.

cc/layers/scrollbar_layer_impl.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/layers/scrollbar_layer_impl.h"
 
+#include <algorithm>
+
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/quad_sink.h"
 #include "cc/quads/solid_color_draw_quad.h"
 #include "cc/quads/texture_draw_quad.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "cc/trees/layer_tree_settings.h"
 #include "ui/gfx/rect_conversions.h"
 
 namespace cc {
@@ skipping 217 matching lines @@
   if (orientation_ == VERTICAL)
     track_length += vertical_adjust_;
 
   if (layer_tree_impl()->settings().solid_color_scrollbars) {
     thumb_length = std::max(
         static_cast<int>(visible_to_total_length_ratio_ * track_length),
         thumb_thickness_);
   }
 
   // With the length known, we can compute the thumb's position.
-  float ratio = current_pos_ / maximum_;
+  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) + track_start_;
 
   gfx::RectF thumb_rect;
   if (orientation_ == HORIZONTAL) {
     thumb_rect = gfx::RectF(thumb_offset, vertical_adjust_,
                             thumb_length, thumb_thickness_);
   } else {
     thumb_rect = gfx::RectF(0.f, thumb_offset,
                             thumb_thickness_, thumb_length);
   }
 
   return ScrollbarLayerRectToContentRect(thumb_rect);
 }
 
 void ScrollbarLayerImpl::DidLoseOutputSurface() {
   track_resource_id_ = 0;
   thumb_resource_id_ = 0;
 }
 
 const char* ScrollbarLayerImpl::LayerTypeAsString() const {
   return "cc::ScrollbarLayerImpl";
 }
 
 }  // namespace cc