Guard against division by zero for useless scroll bars.

ui/views/controls/scrollbar/scrollbar_unittest.cc

 // Copyright (c) 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 "build/build_config.h"
 #include "ui/views/controls/scrollbar/native_scroll_bar.h"
 #include "ui/views/controls/scrollbar/native_scroll_bar_views.h"
 #include "ui/views/controls/scrollbar/scroll_bar.h"
 #include "ui/views/test/views_test_base.h"
 #include "ui/views/widget/widget.h"
@@ skipping 157 matching lines @@
   // Scroll to the end of the content.
   scrollbar_->Update(100, 1001, 0);
   EXPECT_TRUE(scrollbar_->ScrollByContentsOffset(-1));
   // Shrink and then re-expand the content.
   scrollbar_->Update(100, 1000, 0);
   scrollbar_->Update(100, 1001, 0);
   // Ensure the scrollbar allows scrolling to the end.
   EXPECT_TRUE(scrollbar_->ScrollByContentsOffset(-1));
 }
 
+TEST_F(NativeScrollBarTest, ThumbFullLengthOfTrack) {
+  // Shrink content so that it fits within the viewport.
+  scrollbar_->Update(100, 10, 0);
+  EXPECT_EQ(scrollbar_->GetTrackBounds().width(),
+            scrollbar_->GetThumbSizeForTest());
+  // Emulate a click on the full size scroll bar.
+  scrollbar_->ScrollToThumbPosition(0, false);
+  EXPECT_EQ(0, scrollbar_->GetPosition());
+  // Emulate a key down.
+  scrollbar_->ScrollByAmount(BaseScrollBar::SCROLL_NEXT_LINE);
+  EXPECT_EQ(0, scrollbar_->GetPosition());
+
+  // Expand content so that it fits *exactly* within the viewport.
+  scrollbar_->Update(100, 100, 0);
+  EXPECT_EQ(scrollbar_->GetTrackBounds().width(),
+            scrollbar_->GetThumbSizeForTest());
+  // Emulate a click on the full size scroll bar.
+  scrollbar_->ScrollToThumbPosition(0, false);
+  EXPECT_EQ(0, scrollbar_->GetPosition());
+  // Emulate a key down.
+  scrollbar_->ScrollByAmount(BaseScrollBar::SCROLL_NEXT_LINE);
+  EXPECT_EQ(0, scrollbar_->GetPosition());
+}
+
 }  // namespace views