[css-overflow-3] Add some guidance about mapping user input direction to scrolling in the presence of transforms. w3c#5733

Author: fantasai

css-overflow-3/Overview.bs

@@ -452,6 +452,20 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 		with a descendent of the ''visibility: hidden'' [=scroll container=]
 		that is itself set to ''visibility: visible''.
 
+	Note: In the case where the [=scroll container=] (or one of its ancestors)
+	is the target of a graphical transform,
+	the UA might need to take this transform into account
+	when mapping user inputs to scrolling operations.
+	For instance, on a touch screen where the user scrolls
+	by directly dragging the content,
+	the transform would be expected to be taken into account
+	to match the direction of scrolling to the gesture.
+	On the other hand, other user inputs
+	(such as the Page Down key, or a 1D scroll wheel)
+	might be more naturally interpreted ignoring the transform.
+	Choosing the appropriate behavior for each scrolling mechanism
+	is the responsibility of the UA.
+
 <h3 id="static-media">
 Overflow in Print and Other Static Media</h3>