Block the scroll elements from propagating drag events up the element chain

In particular, this caused an issue where we could "drag" through the scroll track, which could lead to selecting text in a parent textarea.

Source/Core/ElementScroll.cpp

@@ -210,6 +210,7 @@ bool ElementScroll::CreateScrollbar(Orientation orientation)
 		Factory::InstanceElement(element, "*", orientation == VERTICAL ? "scrollbarvertical" : "scrollbarhorizontal", XMLAttributes());
 	scrollbars[orientation].element = scrollbar_element.get();
 	scrollbars[orientation].element->SetProperty(PropertyId::Clip, Property(1, Unit::NUMBER));
+	scrollbars[orientation].element->SetProperty(PropertyId::Drag, Property(Style::Drag::Block));
 
 	scrollbars[orientation].widget = MakeUnique<WidgetScroll>(scrollbars[orientation].element);
 	scrollbars[orientation].widget->Initialise(orientation == VERTICAL ? WidgetScroll::VERTICAL : WidgetScroll::HORIZONTAL);
@@ -229,6 +230,7 @@ bool ElementScroll::CreateCorner()
 	ElementPtr corner_element = Factory::InstanceElement(element, "*", "scrollbarcorner", XMLAttributes());
 	corner = corner_element.get();
 	corner->SetProperty(PropertyId::Clip, Property(1, Unit::NUMBER));
+	corner->SetProperty(PropertyId::Drag, Property(Style::Drag::Block));
 
 	Element* child = element->AppendChild(std::move(corner_element), false);
 	UpdateScrollElementProperties(child);