[css-round-display] Add issue about positioning elements based on the edge of the containing block

Author: Jihye Hong

css-round-display/Overview.bs

@@ -379,3 +379,6 @@ How to position items to the edge of the containing block without overflowing it
 <p class="issue">
 One of the profits of polar coordinate system is performance improvement. The significant performance improvement can be seen in animation using the polar coordinate. How to define animation in polar coordinate? Is there any difference between the conventional coordinate and polar coordinate when animate elements?
 </p>
+<p class="issue">
+By 'polar-distance' property, a position of an element is specified based on the containing block's center. In other way, is the method of positioning elements by the distance based on the edge of the containing block needed?
+</p>