[css-overflow-3] Clarify effects of clipping on scrollable overflow #8607

Author: fantasai

css-overflow-3/Overview.bs

@@ -633,19 +633,19 @@ Expanding Clipping Bounds: the 'overflow-clip-margin' property</h3>
 <h3 id=scrollable-overflow-calculation>
 Calculating the Scrollable Overflow Area</h3>
 
-	The <a>scrollable overflow area</a> of a [=scroll container=] is the union of:
+	The <a>scrollable overflow area</a> of a box is the union of:
 	<ul>
 		<li>
-			The [=scroll container=]’s own [=padding box=].
+			Its own [=padding box=].
 
 		<li>
-			All [=line boxes=] directly contained by the [=scroll container=].
+			All [=line boxes=] it directly contains.
 
 		<li>
 			The border boxes
 			of all boxes for which it is the containing block
 			and whose border boxes are positioned not wholly
-			in the [=unreachable scrollable overflow region=],
+			within its [=unreachable scrollable overflow region=] (if any),
 			accounting for transforms by projecting each box onto
 			the plane of the element that establishes its <a>3D rendering context</a>.
 			[[!CSS3-TRANSFORMS]]
@@ -671,17 +671,14 @@ Calculating the Scrollable Overflow Area</h3>
 
 		<li>
 			The <a>scrollable overflow areas</a> of all of the above boxes
-			(including zero-area boxes
-			and accounting for transforms as described above),
-			provided they themselves have ''overflow: visible''
-			(i.e. do not themselves trap the overflow)
-			and that <a>scrollable overflow</a> is not already clipped
-			(e.g. by the 'clip' property or the 'contain' property).
-
-			Note: The 'mask-*' properties [[!CSS-MASKING-1]]
-			do not affect the <a>scrollable overflow area</a>.
+			(including zero-area boxes),
+			clipped to their [=overflow clip edge=]
+			if 'overflow' is not ''overflow/visible'' or 'contain' applies ''contain/paint'',
+			and accounting for transforms as described above.
 
-			ISSUE(8607): Need to evaluate what should/should not clip [=scrollable overflow=].
+			Note: The 'clip', 'clip-path', and 'mask-*' properties [[!CSS-MASKING-1]]
+			do not affect the <a>scrollable overflow area</a>.
+			Only effects that clip to the [=overflow clip edge=] are taken into account.
 
 		<li>
 			Additional padding added