Have Blink be told of scrollbar changes rather than paying attention itself (Chromium side).

content/browser/theme_helper_mac.mm

+// Copyright 2013 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 "content/browser/theme_helper_mac.h"
+
+#include <Foundation/Foundation.h>
+
+#include "content/common/view_messages.h"
+#include "content/public/browser/browser_thread.h"
+#include "content/public/browser/notification_service.h"
+#include "content/public/browser/notification_types.h"
+#include "content/public/browser/render_process_host.h"
+
+@interface ScrollbarPrefsObserver : NSObject
+
++ (void)registerAsObserver;
++ (void)appearancePrefsChanged:(NSNotification*)notification;
++ (void)behaviorPrefsChanged:(NSNotification*)notification;
++ (void)notifyPrefsChangedWithRedraw:(BOOL)redraw;
+
+@end
+
+@implementation ScrollbarPrefsObserver
+
++ (void)registerAsObserver {
+  [[NSDistributedNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(appearancePrefsChanged:)
+             name:@"AppleAquaScrollBarVariantChanged"
+           object:nil
+suspensionBehavior:NSNotificationSuspensionBehaviorDeliverImmediately];
+
+  [[NSDistributedNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(behaviorPrefsChanged:)
+             name:@"AppleNoRedisplayAppearancePreferenceChanged"
+           object:nil
+suspensionBehavior:NSNotificationSuspensionBehaviorCoalesce];
+}
+
++ (void)appearancePrefsChanged:(NSNotification*)notification {
+  [self notifyPrefsChangedWithRedraw:YES];
+}
+
++ (void)behaviorPrefsChanged:(NSNotification*)notification {
+  [self notifyPrefsChangedWithRedraw:NO];
+}
+
++ (void)notifyPrefsChangedWithRedraw:(BOOL)redraw {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
+  [defaults synchronize];
+
+  content::ThemeHelperMac::SendThemeChangeToAllRenderers(
+      [defaults floatForKey:@"NSScrollerButtonDelay"],
+      [defaults floatForKey:@"NSScrollerButtonPeriod"],
+      [defaults boolForKey:@"AppleScrollerPagingBehavior"],
+      redraw);
+}
+
+@end
+
+namespace content {
+
+ThemeHelperMac::ThemeHelperMac() {
+  [ScrollbarPrefsObserver registerAsObserver];
+  registrar_.Add(this,
+                 NOTIFICATION_RENDERER_PROCESS_CREATED,
+                 NotificationService::AllSources());
+}
+
+ThemeHelperMac::~ThemeHelperMac() {
+}
+
+// static
+ThemeHelperMac* ThemeHelperMac::GetInstance() {
+  return Singleton<ThemeHelperMac,
+      LeakySingletonTraits<ThemeHelperMac> >::get();
+}
+
+
+void ThemeHelperMac::Observe(int type,
+                             const NotificationSource& source,
+                             const NotificationDetails& details) {
+  DCHECK_EQ(NOTIFICATION_RENDERER_PROCESS_CREATED, type);
+
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
+  [defaults synchronize];
+
+  RenderProcessHost* rph = Source<RenderProcessHost>(source).ptr();
+  rph->Send(new ViewMsg_UpdateScrollbarTheme(
+      [defaults floatForKey:@"NSScrollerButtonDelay"],
+      [defaults floatForKey:@"NSScrollerButtonPeriod"],
+      [defaults boolForKey:@"AppleScrollerPagingBehavior"],
+      false));
+}
+
+// static
+void ThemeHelperMac::SendThemeChangeToAllRenderers(
+    float initial_button_delay,
+    float autoscroll_button_delay,
+    bool jump_on_track_click,
+    bool redraw) {
+  for (RenderProcessHost::iterator it(RenderProcessHost::AllHostsIterator());
+       !it.IsAtEnd();
+       it.Advance()) {
+    it.GetCurrentValue()->Send(new ViewMsg_UpdateScrollbarTheme(
+        initial_button_delay,
+        autoscroll_button_delay,
+        jump_on_track_click,
+        redraw));
+  }
+}
+
+}  // namespace content