[css-contain] certain non-local paint effects affect paint optimizations

Solves w3c#6325

Author: frivoal

css-contain-1/Overview.bs

@@ -1164,9 +1164,17 @@ Paint Containment</h3>
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 
 	1. If the [=paint containment box|containment box=] is off-screen or obscured,
-		the UA can directly skip trying to paint its contents,
+		the UA can usually skip trying to paint its contents,
 		as they're guaranteed to be off-screen/obscured as well.
 
+		Note: Some paint effects such as the ''blur()'' filter from [[FILTER-EFFECTS-1]]
+		have non local effects.
+		The user agent needs to keep track of these,
+		as it may need to repaint parts of an element with such a filter
+		when its descendents change,
+		even if they have [=paint containment=]
+		and could otherwise be skipped.
+
 	2. Unless the clipped content is made accessible via a separate mechanism
 		such as the 'overflow', 'resize', or 'text-overflow' properties,
 		the UA can reserve "canvas" space for the box exactly the box's size.
@@ -1287,6 +1295,9 @@ This appendix is <em>informative</em>.
 
 				<li>
 					terminology change: replace "containing box" with "containment box"
+
+				<li>
+					Note that paint effects with non-local effects can limit certain optimization opportunities.
 			</ul>
 	</ul>
 

css-contain-2/Overview.bs

@@ -1133,9 +1133,17 @@ Possible Paint-Containment Optimizations</h4>
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 
 	1. If the [=paint containment box|containment box=] is off-screen or obscured,
-		the UA can directly skip trying to paint its contents,
+		the UA can usually skip trying to paint its contents,
 		as they're guaranteed to be off-screen/obscured as well.
 
+		Note: Some paint effects such as the ''blur()'' filter from [[FILTER-EFFECTS-1]]
+		have non local effects.
+		The user agent needs to keep track of these,
+		as it may need to repaint parts of an element with such a filter
+		when its descendents change,
+		even if they have [=paint containment=]
+		and could otherwise be skipped.
+
 	2. Unless the clipped content is made accessible via a separate mechanism
 		such as the 'overflow', 'resize', or 'text-overflow' properties,
 		the UA can reserve "canvas" space for the box exactly the box's size.
@@ -1982,6 +1990,7 @@ Changes from <a href="https://www.w3.org/TR/2020/WD-css-contain-2-20201216/">202
 * Clarify that scrollIntoView() doesn't scroll to children of a ''content-visibility:hidden'' element
 * Defined that elements having an ancestor with ''content-visibility: hidden'' don't generate boxes in the top layer
 * Defined that being in the top-layer makes an element relevant to the user
+* Noted that paint effects with non-local effects can limit certain optimization opportunities.
 
 <h3 id="changes-since-2020-06-03">
 Changes from <a href="https://www.w3.org/TR/2020/WD-css-contain-2-20200603/">2020-06-03 Working Draft</a>