[css-grid-1] Add previous change to Changes section.

Author: tabatkins

css-grid-1/Overview.bs

@@ -4633,6 +4633,17 @@ Acknowledgements</h2>
 	Rossen Atanassov, Alex Mogilevsky, Phil Cupp, and Peter Salas of Microsoft for creating the initial proposal.
 	Thanks also to Eliot Graff for editorial input.
 
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 <h2 class="no-num" id="changes">
 Changes</h2>
 
@@ -5146,6 +5157,17 @@ Clarifications</h4>
 				e.g. when it is [=replaced element|replaced=]
 				or in a ''display: none'' subtree.</ins>
 			</blockquote>
+
+		<li id="change-2017-flexible-tracks">
+			Clarified antecedents for flexible tracks
+			in [[#algo-flex-tracks]].
+
+			<blockquote>
+				The used flex fraction is the maximum of:
+
+				* <ins>For each flexible track,</ins> if the track’s flex factor is greater than one, the result of dividing the track’s base size by its flex factor; otherwise, the track’s base size.
+				* <ins>For each grid item that crosses a flexible track,</ins> the result of finding the size of an fr using all the grid tracks that the item crosses and a space to fill of the item’s max-content contribution.
+			</blockquote>
 	</ul>
 
 <h3 id="changes-2016">