[css-grid-1][css-grid-2] Allow fixed-range tracks to absorb leftover space from spanning items when intrinsic-range tracks are filled up, if possible. w3c#3648

Author: tabatkins

css-grid-1/Overview.bs

@@ -4399,6 +4399,18 @@ Distributing Extra Space Across Spanned Tracks</h4>
 						and the track is not marked [=infinitely growable=],
 						then each |item-incurred increase| will be zero.
 
+					<li>
+						<strong>Distribute |space| to non-affected tracks:</strong>
+
+						If extra |space| remains at this point,
+						and the item spans both |affected tracks| and non-|affected tracks|,
+						distribute space as for the previous step,
+						but into the non-|affected tracks| instead.
+
+						Note: This distributes any remaining space into tracks
+						that have not yet reached their growth limits,
+						instead of violating the growth limits of the |affected tracks|.
+
 					<li>
 						<strong>Distribute |space| beyond limits:</strong>
 

css-grid-2/Overview.bs

@@ -5074,6 +5074,18 @@ Distributing Extra Space Across Spanned Tracks</h4>
 						and the track is not marked [=infinitely growable=],
 						then each |item-incurred increase| will be zero.
 
+					<li>
+						<strong>Distribute |space| to non-affected tracks:</strong>
+
+						If extra |space| remains at this point,
+						and the item spans both |affected tracks| and non-|affected tracks|,
+						distribute space as for the previous step,
+						but into the non-|affected tracks| instead.
+
+						Note: This distributes any remaining space into tracks
+						that have not yet reached their growth limits,
+						instead of violating the growth limits of the |affected tracks|.
+
 					<li>
 						<strong>Distribute |space| beyond limits:</strong>