[css-flexbox] Specify the heuristic for finding the intrinsic width of a multiline column flexbox.

Author: tabatkins

css-flexbox/Overview.bs

@@ -2765,26 +2765,17 @@ Intrinsic Sizes</h3>
 	are the sum of the flex line cross sizes
 	resulting from sizing the flex container under a <a>cross-axis</a> <a>min-content constraint</a>/<a>max-content constraint</a>, respectively.
 
-	Issue: Current browsers set the flex container’s inner size
-	to the size of the largest min-content contribution,
-	instead of summing the lines.
-	This is clearly wrong,
-	as it is guaranteed to cause overflow if there are multiple lines.
-
-	<div class="issue">
-		In an ideal world, the multi-line cross size would be calculated by, one by one,
-		laying out elements with the constraint applied into the flex lines,
-		and whenever a new largest cross-size in a line is found,
-		redoing layout for all previous items in the line to match that width
-		(so you get accurate line-breaking).
-
-		Current browsers perform an approximation of this,
-		finding the largest intrinsic cross-size among *all* items
-		and then shrink-to-fit sizing everything into that much available space.
-		Is this approximation sufficiently useful to specify?
-		Can we do better?
-	</div>
-
+	For the purposes of the preceding paragraph,
+	if the <a>flex container</a> is ''flex-flow: column wrap;'',
+	then it's sized by first finding the largest <a lt="min-content contribution">min-content</a> or <a>max-content contribution</a>
+	as appropriate,
+	among the <a>flex items</a>,
+	then using that size as the <a>available space</a> in the <a>cross axis</a> for all of the <a>flex items</a> during layout.
+
+	Note: This gives a reasonable approximation of the size that the flex container should be.
+	It's not a <em>perfect</em> fit in some degenerate cases,
+	but doing it completely correct is insanely expensive,
+	and this works reasonably well.
 
 	The <strong>main-size <a lt="min-content contribution">min-content</a>/<a>max-content contribution</a> of a <a>flex item</a></strong>
 	is its outer <a lt="min-content size">min-content</a>/<a>max-content size</a>,