Handle the scroll wheel changing over scrollbars.

* Extract the scrollbar change based on the mouse wheel delta into a protected
  method of BView.
* Call that method from BScrollBar's MessageReceived.

With this change it is now a bit easier to scroll horizontally around the
system by putting the mouse cursor over a horizontal scrollbar and using the
wheel.

Fixes #8631.

headers/os/interface/View.h

@@ -584,6 +584,8 @@ protected:
 
 	virtual	void				LayoutChanged();
 
+			void				ScrollWithMouseWheelDelta(BScrollBar*, float);
+
 private:
 			void				_Layout(bool force, BLayoutContext* context);
 			void				_LayoutLeft(BLayout* deleted);

src/kits/interface/ScrollBar.cpp

@@ -642,6 +642,22 @@ BScrollBar::MessageReceived(BMessage* message)
 				ValueChanged(value);
 			break;
 		}
+		case B_MOUSE_WHEEL_CHANGED:
+		{
+			// Must handle this here since BView checks for the existence of
+			// scrollbars, which a scrollbar itself does not have
+			float deltaX = 0.0f, deltaY = 0.0f;
+			message->FindFloat("be:wheel_delta_x", &deltaX);
+			message->FindFloat("be:wheel_delta_y", &deltaY);
+
+			if (deltaX == 0.0f && deltaY == 0.0f)
+				break;
+
+			if (deltaX != 0.0f && deltaY == 0.0f)
+				deltaY = deltaX;
+
+			ScrollWithMouseWheelDelta(this, deltaY);
+		}
 		default:
 			BView::MessageReceived(message);
 			break;

src/kits/interface/View.cpp

@@ -4350,31 +4350,12 @@ BView::MessageReceived(BMessage* msg)
 				if (deltaX == 0.0f && deltaY == 0.0f)
 					break;
 
-				float smallStep, largeStep;
 				if (horizontal != NULL) {
-					horizontal->GetSteps(&smallStep, &largeStep);
+					ScrollWithMouseWheelDelta(horizontal, deltaX);
-
-					// pressing the option/command/control key scrolls faster
-					if (modifiers()
-						& (B_OPTION_KEY | B_COMMAND_KEY | B_CONTROL_KEY)) {
-						deltaX *= largeStep;
-					} else
-						deltaX *= smallStep * 3;
-
-					horizontal->SetValue(horizontal->Value() + deltaX);
 				}
 
 				if (vertical != NULL) {
-					vertical->GetSteps(&smallStep, &largeStep);
+					ScrollWithMouseWheelDelta(vertical, deltaY);
-
-					// pressing the option/command/control key scrolls faster
-					if (modifiers()
-						& (B_OPTION_KEY | B_COMMAND_KEY | B_CONTROL_KEY)) {
-						deltaY *= largeStep;
-					} else
-						deltaY *= smallStep * 3;
-
-					vertical->SetValue(vertical->Value() + deltaY);
 				}
 				break;
 			}
@@ -5721,6 +5702,26 @@ BView::_SwitchServerCurrentView() const
 }
 
 
+void
+BView::ScrollWithMouseWheelDelta(BScrollBar* scrollBar, float delta)
+{
+	if (scrollBar == NULL || delta == 0.0f)
+		return;
+
+	float smallStep, largeStep;
+	scrollBar->GetSteps(&smallStep, &largeStep);
+
+	// pressing the option/command/control key scrolls faster
+	if (modifiers()
+			& (B_OPTION_KEY | B_COMMAND_KEY | B_CONTROL_KEY)) {
+		delta *= largeStep;
+	} else
+		delta *= smallStep * 3;
+
+	scrollBar->SetValue(scrollBar->Value() + delta);
+}
+
+
 #if __GNUC__ == 2