Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp

Index: Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp
===================================================================
--- Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp	(revision 166567)
+++ Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp	(working copy)
@@ -145,9 +145,42 @@
 
 bool ScrollbarThemeGtkOrAura::shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent& evt)
 {
-    return (evt.shiftKey() && evt.button() == LeftButton) || (evt.button() == MiddleButton);
+#if defined(TOOLKIT_GTK)

[Elliot Glaysher, 2014/02/11 18:36:56]

You probably want defined(TOOLKIT_GTK) || (defined(OS_LINUX) &&
!defined(OS_CHROMEOS)) here.

(I'm fine experimenting with turning on the snapback behaviour on linux aura for
now, since qt does it even if gtk doesn't.)

[Peter Kasting, 2014/02/12 03:17:58]

Done.

+    if (evt.button() == MiddleButton)
+        return true;
+#endif
+    return (evt.shiftKey() && evt.button() == LeftButton);
 }
 
+bool ScrollbarThemeGtkOrAura::shouldSnapBackToDragOrigin(ScrollbarThemeClient* scrollbar, const PlatformMouseEvent& evt)
+{
+#if defined(TOOLKIT_GTK)

[sky, 2014/02/10 15:14:30]

Don't you want !win here? Otherwise won't chromeos pick up this behavior?

[Peter Kasting, 2014/02/10 23:09:16]

To re-quote my initial comment on this CL:

"(I chose "not GTK" as the condition because AFAIK we want CrOS behavior to
match Windows behavior, and thus OS(WIN) wouldn't be the right #define to look
at.)"

Am I correct that in general we want CrOS to be copying Windows behavior and not
GTK behavior?  Classically I've always tried to port Windows' small behavioral
details over to CrOS.

+    return false;
+#endif
+
+    // Constants used to figure the drag rect outside which we should snap the
+    // scrollbar thumb back to its origin. These calculations are based on
+    // observing the behavior of the MSVC8 main window scrollbar + some
+    // guessing/extrapolation.
+    static const int kOffEndMultiplier = 3;
+    static const int kOffSideMultiplier = 8;
+
+    // Find the rect within which we shouldn't snap, by expanding the track rect
+    // in both dimensions.
+    IntRect rect = trackRect(scrollbar);
+    const bool horz = scrollbar->orientation() == HorizontalScrollbar;
+    const int thickness = scrollbarThickness(scrollbar->controlSize());
+    rect.inflateX((horz ? kOffEndMultiplier : kOffSideMultiplier) * thickness);
+    rect.inflateY((horz ? kOffSideMultiplier : kOffEndMultiplier) * thickness);
+
+    // Convert the event to local coordinates.
+    IntPoint mousePosition = scrollbar->convertFromContainingWindow(evt.position());
+    mousePosition.move(scrollbar->x(), scrollbar->y());
+
+    // We should snap iff the event is outside our calculated rect.
+    return !rect.contains(mousePosition);
+}
+
 IntSize ScrollbarThemeGtkOrAura::buttonSize(ScrollbarThemeClient* scrollbar)
 {
     if (scrollbar->orientation() == VerticalScrollbar) {