Fix event targeting for overlay scrollbar thumbs (in native UI).

ui/views/controls/scrollbar/overlay_scroll_bar.cc

Index: ui/views/controls/scrollbar/overlay_scroll_bar.cc
diff --git a/ui/views/controls/scrollbar/overlay_scroll_bar.cc b/ui/views/controls/scrollbar/overlay_scroll_bar.cc
index de3b193e1760171e3f277a681fa73b7017d0657c..563a129e82284a2aad7f5598eeec2611c21e4d76 100644
--- a/ui/views/controls/scrollbar/overlay_scroll_bar.cc
+++ b/ui/views/controls/scrollbar/overlay_scroll_bar.cc
@@ -19,7 +19,7 @@ namespace {
 const int kThumbThickness = 11;
 // When hovered, the thumb takes up the full width. Otherwise, it's a bit
 // slimmer.
-const int kThumbUnhoveredDifference = 4;
+const int kThumbHoverOffset = 4;
 const int kThumbStroke = 1;
 const float kThumbHoverAlpha = 0.5f;
 const float kThumbDefaultAlpha = 0.3f;
@@ -28,16 +28,27 @@ const float kThumbDefaultAlpha = 0.3f;
 
 OverlayScrollBar::Thumb::Thumb(OverlayScrollBar* scroll_bar)
     : BaseScrollBarThumb(scroll_bar), scroll_bar_(scroll_bar) {
+  // |scroll_bar| isn't done being constructed; it's not safe to do anything
+  // that might reference it yet.
+}
+
+OverlayScrollBar::Thumb::~Thumb() {}
+
+void OverlayScrollBar::Thumb::Init() {
   SetPaintToLayer(true);
+  layer()->SetFillsBoundsOpaquely(false);
   // Animate all changes to the layer except the first one.
   OnStateChanged();
   layer()->SetAnimator(ui::LayerAnimator::CreateImplicitAnimator());
 }
 
-OverlayScrollBar::Thumb::~Thumb() {}
-
 gfx::Size OverlayScrollBar::Thumb::GetPreferredSize() const {
-  return gfx::Size(kThumbThickness, kThumbThickness);
+  // The visual size of the thumb is kThumbThickness, but it slides back and
+  // forth by kThumbHoverOffset. To make event targetting work well, expand the
+  // width of the thumb such that it's always taking up the full width of the
+  // track regardless of the offset.
+  return gfx::Size(kThumbThickness + kThumbHoverOffset,
+                   kThumbThickness + kThumbHoverOffset);
 }
 
 void OverlayScrollBar::Thumb::OnPaint(gfx::Canvas* canvas) {
@@ -45,6 +56,8 @@ void OverlayScrollBar::Thumb::OnPaint(gfx::Canvas* canvas) {
   fill_paint.setStyle(SkPaint::kFill_Style);
   fill_paint.setColor(SK_ColorBLACK);
   gfx::RectF fill_bounds(GetLocalBounds());
+  fill_bounds.Inset(gfx::InsetsF(IsHorizontal() ? kThumbHoverOffset : 0,
+                                 IsHorizontal() ? 0 : kThumbHoverOffset, 0, 0));
   fill_bounds.Inset(gfx::InsetsF(kThumbStroke, kThumbStroke,
                                  IsHorizontal() ? 0 : kThumbStroke,
                                  IsHorizontal() ? kThumbStroke : 0));
@@ -55,7 +68,7 @@ void OverlayScrollBar::Thumb::OnPaint(gfx::Canvas* canvas) {
   stroke_paint.setColor(SK_ColorWHITE);
   stroke_paint.setStrokeWidth(kThumbStroke);
   gfx::RectF stroke_bounds(fill_bounds);
-  stroke_bounds.Inset(gfx::InsetsF(-0.5f));
+  stroke_bounds.Inset(gfx::InsetsF(kThumbStroke / 2.f));
   // The stroke doesn't apply to the far edge of the thumb.
   SkPath path;
   path.moveTo(gfx::PointFToSkPoint(stroke_bounds.top_right()));
@@ -82,8 +95,8 @@ void OverlayScrollBar::Thumb::OnStateChanged() {
   if (GetState() == CustomButton::STATE_NORMAL) {
     gfx::Transform translation;
     translation.Translate(
-        gfx::Vector2d(IsHorizontal() ? 0 : kThumbUnhoveredDifference,
-                      IsHorizontal() ? kThumbUnhoveredDifference : 0));
+        gfx::Vector2d(IsHorizontal() ? 0 : kThumbHoverOffset,
+                      IsHorizontal() ? kThumbHoverOffset: 0));
     layer()->SetTransform(translation);
     layer()->SetOpacity(kThumbDefaultAlpha);
 
@@ -96,18 +109,24 @@ void OverlayScrollBar::Thumb::OnStateChanged() {
 }
 
 OverlayScrollBar::OverlayScrollBar(bool horizontal)
-    : BaseScrollBar(horizontal, new Thumb(this)), hide_timer_(false, false) {
+    : BaseScrollBar(horizontal), hide_timer_(false, false) {
+  auto thumb = new Thumb(this);
+  SetThumb(thumb);
+  thumb->Init();
   set_notify_enter_exit_on_child(true);
   SetPaintToLayer(true);
   layer()->SetMasksToBounds(true);
   layer()->SetFillsBoundsOpaquely(false);
 }
 
-OverlayScrollBar::~OverlayScrollBar() {
-}
+OverlayScrollBar::~OverlayScrollBar() {}
 
 gfx::Rect OverlayScrollBar::GetTrackBounds() const {
-  return GetLocalBounds();
+  gfx::Rect local = GetLocalBounds();
+  // The track has to be wide enough for the thumb.
+  local.Inset(gfx::Insets(IsHorizontal() ? -kThumbHoverOffset : 0,
+                          IsHorizontal() ? 0 : -kThumbHoverOffset, 0, 0));
+  return local;
 }
 
 int OverlayScrollBar::GetLayoutSize() const {