[css-flexbox] Revert the changes from #1735, but re-apply the adjusted opening paragraph wording, and add some notes about the behavior so we remember our intentions in the future.

Author: tabatkins

css-flexbox/Overview.bs

@@ -2789,55 +2789,34 @@ Flex Container Intrinsic Main Sizes</h4>
 
 	The <strong><a>max-content</a> <a>main size</a> of a <a>flex container</a></strong>
 	is the smallest size the <a>flex container</a> can take
-	while maintaining the [[#intrinsic-item-contributions|max-content contributions]] of its <a>flex items</a> insofar as possible
-	(but also respecting continuity with zero for 0 < ∑<var>flex factors</var> < 1):
+	while maintaining the [[#intrinsic-item-contributions|max-content contributions]] of its <a>flex items</a>,
+	insofar as allowed by the items’ own flexibility:
 
 	<ol>
 		<li>
-			Place all <a>flex items</a> into lines of infinite length.
+			For each <a>flex item</a>,
+			subtract its outer <a>flex base size</a> from its [[#intrinsic-item-contributions|max-content contribution]] size.
+			If that result is not zero, divide it by (if the result was positive) its <a>flex grow factor</a> floored at 1,
+			or (if the result was negative) by its <a>scaled flex shrink factor</a>, having floored the <a>flex shrink factor</a> at 1.
+			This is the item's <var>max-content flex fraction</var>.
 
 		<li>
-			For each <a>flex item</a>,
-			find its <var>max-content flex fraction</var>:
-			Subtract its outer <a>flex base size</a> from its [[#intrinsic-item-contributions|max-content contribution]] size.
-			If that result is positive,
-			divide by its <a>flex grow factor</a> floored at 1;
-			if negative,
-			by its <a>flex shrink factor</a> floored at 1
-			multipled by its <var>flex base ratio</var>--
-			the ratio of its inner <a>flex base size</a>
-			to the sum of the inner <a>flex base sizes</a>
-			of all shrinkable items on the line.
+			Place all <a>flex items</a> into lines of infinite length.
 
 		<li>
 			Within each line,
 			find the largest <var>max-content flex fraction</var>
 			among all the <a>flex items</a>.
-
-		<li>
-			If the sum of the <a>flex grow factors</a>
-			(<a>flex shrink factors</a>,
-			if the largest <var>max-content flex fraction</var> was negative)
-			on a line is non-zero but less than one,
-			divide each <a>flex grow factor</a> (<a>flex shrink factor</a>)
-			on that line
-			by their sum to rescale their total to one.
-
-		<li>
-			Add each item’s outer <a>flex base size</a>
-			to the product of its <em>rescaled</em> <a>flex grow factor</a>
-			(or its <em>rescaled</em> <a>flex shrink factor</a>
-			multiplied by its <var>flex base ratio</var>,
-			if the chosen <var>max-content flex fraction</var> was negative)
+			Add each item’s <a>flex base size</a>
+			to the product of its <a>flex grow factor</a>
+			(or <a>scaled flex shrink factor</a>, if the chosen <var>max-content flex fraction</var> was negative)
 			and the chosen <var>max-content flex fraction</var>,
-			then clamp that resulting item size according to the item’s
-			<a lt="max main size property">max</a> and
-			<a>min main size properties</a>.
+			then clamp that result by the <a>max main size property</a>
+			floored by the <a>min main size property</a>.
 
 		<li>
 			The <a>flex container</a>’s <a>max-content size</a> is the
-			largest sum of the afore-calculated outer sizes
-			of all items within a single line.
+			largest sum of the afore-calculated sizes of all items within a single line.
 	</ol>
 
 	The <strong><a>min-content</a> <a>main size</a></strong> of a <em><a>single-line</a></em> flex container
@@ -2848,6 +2827,40 @@ Flex Container Intrinsic Main Sizes</h4>
 	it is simply the largest [[#intrinsic-item-contributions|min-content contribution]]
 	of all the <a>flex items</a> in the <a>flex container</a>.
 
+	<details class=note>
+		<summary>Implications of this algorithm when the sum of flex is less than 1</summary>
+
+		The above algorithm is designed to give the correct behavior for two cases in particular,
+		and make the <a>flex container’s</a> size continuous as you transition between the two:
+
+		1. If all items are inflexible,
+			the flexbox is sized to the sum of their <a>flex base size</a>.
+			(An inflexible <a>flex base size</a> basically substitutes for a 'width'/'height',
+			which, when specified, is what a <a>max content contribution</a> is based on in Block Layout.)
+
+		2. When all items are flexible with flexes > 1,
+			the flexbox is sized to the sum of the <a>max content contributions</a> of its items
+			(or perhaps a slightly larger size,
+			so that every flex item is <em>at least</em> the size of its <a>max-content contribution</a>,
+			but also has the correct ratio of its size to the size of the other items,
+			as determined by its flexibility).
+
+		For example, if a <a>flex container</a> has a single <a>flex item</a>
+		with ''flex-basis: 100px;'' but a max-content size of ''200px'',
+		then when the item is ''flex-grow: 0'', the flexbox (and flex item) is ''100px'' wide,
+		but when the item is ''flex-grow: 1'' or higher, the flexbox (and flex item) is ''200px'' wide.
+
+		There are several possible ways to make the overall behavior continuous between these two cases,
+		particularly when the sum of flexibilities on a line is between 0 and 1,
+		but all of them have drawbacks.
+		We chose one we feel has the least bad implications;
+		unfortunately, it "double-applies" the flexibility when the sum of the flexibilities is less than 1.
+		In the above example, if the item has ''flex-grow: .5'',
+		then the flexbox ends up ''150px'' wide,
+		but the item then sizes normally into that available space,
+		ending up ''125px'' wide.
+	</details>
+
 <h4 id='intrinsic-cross-sizes'>
 Flex Container Intrinsic Cross Sizes</h4>