Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp

 /*
  * Copyright (c) 2008, 2009, Google 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:
  *
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@@ skipping 127 matching lines @@
         state = blink::WebThemeEngine::StatePressed;
     else if (scrollbar->hoveredPart() == ThumbPart)
         state = blink::WebThemeEngine::StateHover;
     else
         state = blink::WebThemeEngine::StateNormal;
     blink::Platform::current()->themeEngine()->paint(canvas, scrollbar->orientation() == HorizontalScrollbar ? blink::WebThemeEngine::PartScrollbarHorizontalThumb : blink::WebThemeEngine::PartScrollbarVerticalThumb, state, blink::WebRect(rect), 0);
 }
 
 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) {
         IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarUpArrow);
         return IntSize(size.width(), scrollbar->height() < 2 * size.height() ? scrollbar->height() / 2 : size.height());
     }
 
     // HorizontalScrollbar
     IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarLeftArrow);
     return IntSize(scrollbar->width() < 2 * size.width() ? scrollbar->width() / 2 : size.width(), size.height());
 }
 
 int ScrollbarThemeGtkOrAura::minimumThumbLength(ScrollbarThemeClient* scrollbar)
 {
     if (scrollbar->orientation() == VerticalScrollbar) {
         IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarVerticalThumb);
         return size.height();
     }
 
     IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarHorizontalThumb);
     return size.width();
 }
 
 } // namespace WebCore