Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarThemeMacCommon.mm

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 56 matching lines @@
     DEFINE_STATIC_LOCAL(ScrollbarSet, set, ());
     return set;
 }
 
 }
 
 namespace WebCore {
 
 static float gInitialButtonDelay = 0.5f;
 static float gAutoscrollButtonDelay = 0.05f;
-static bool gJumpOnTrackClick = false;
 static NSScrollerStyle gPreferredScrollerStyle = NSScrollerStyleLegacy;
 
 ScrollbarTheme* ScrollbarTheme::nativeTheme()
 {
     static ScrollbarThemeMacCommon* theme = NULL;
     if (theme)
         return theme;
     if (ScrollbarThemeMacCommon::isOverlayAPIAvailable()) {
         DEFINE_STATIC_LOCAL(ScrollbarThemeMacOverlayAPI, overlayTheme, ());
         theme = &overlayTheme;
@@ skipping 198 matching lines @@
     IntRect tickmarkTrackRect = rect;
     tickmarkTrackRect.setX(tickmarkTrackRect.x() + 1);
     tickmarkTrackRect.setWidth(tickmarkTrackRect.width() - 2);
     paintGivenTickmarks(context, scrollbar, tickmarkTrackRect, tickmarks);
 }
 
 ScrollbarThemeMacCommon::~ScrollbarThemeMacCommon()
 {
 }
 
-void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, bool jumpOnTrackClick, NSScrollerStyle preferredScrollerStyle, bool redraw)
+void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw)
 {
     updateButtonPlacement();
     gInitialButtonDelay = initialButtonDelay;
     gAutoscrollButtonDelay = autoscrollButtonDelay;
-    gJumpOnTrackClick = jumpOnTrackClick;
     bool sendScrollerStyleNotification = gPreferredScrollerStyle != preferredScrollerStyle;
     gPreferredScrollerStyle = preferredScrollerStyle;
     if (redraw && !scrollbarSet().isEmpty()) {
         ScrollbarSet::iterator end = scrollbarSet().end();
         for (ScrollbarSet::iterator it = scrollbarSet().begin(); it != end; ++it) {
             (*it)->styleChanged();
             (*it)->invalidate();
         }
     }
     if (sendScrollerStyleNotification) {
         [[NSNotificationCenter defaultCenter]
             postNotificationName:@"NSPreferredScrollerStyleDidChangeNotification"
                           object:nil
                         userInfo:@{ @"NSScrollerStyle" : @(gPreferredScrollerStyle) }];
     }
 }
 
 double ScrollbarThemeMacCommon::initialAutoscrollTimerDelay()
 {
     return gInitialButtonDelay;
 }
 
 double ScrollbarThemeMacCommon::autoscrollTimerDelay()
 {
     return gAutoscrollButtonDelay;
 }
 
-bool ScrollbarThemeMacCommon::shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent& evt)
-{
-    if (evt.button() != LeftButton)
-        return false;
-    if (gJumpOnTrackClick)
-        return !evt.altKey();
-    return evt.altKey();
-}
-
 bool ScrollbarThemeMacCommon::shouldDragDocumentInsteadOfThumb(ScrollbarThemeClient*, const PlatformMouseEvent& event)
 {
     return event.altKey();
 }
 
 int ScrollbarThemeMacCommon::scrollbarPartToHIPressedState(ScrollbarPart part)
 {
     switch (part) {
         case BackButtonStartPart:
             return kThemeTopOutsideArrowPressed;
@@ skipping 21 matching lines @@
 // static
 bool ScrollbarThemeMacCommon::isOverlayAPIAvailable()
 {
     static bool apiAvailable =
         [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
         && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
     return apiAvailable;
 }
 
 } // namespace WebCore