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..0d92362c883976ef5114415cbc4faf5116b3237e 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;
@@ -37,7 +37,12 @@ OverlayScrollBar::Thumb::Thumb(OverlayScrollBar* scroll_bar)
 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);

[sadrul, 2016/11/19 01:43:04]

This would affect layout (and thus painting) too, right? Can that be the cause
of the visual glitch varkha@ mentioned in the earlier comment?

 }
 
 void OverlayScrollBar::Thumb::OnPaint(gfx::Canvas* canvas) {
@@ -45,6 +50,8 @@ 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));
@@ -82,8 +89,8 @@ 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);
 
@@ -107,7 +114,11 @@ 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 {