Add comment for StickyContainer extension

src/components/structures/ScrollPanel.js

@@ -117,7 +117,11 @@ export default class ScrollPanel extends StickyContainer {
         debuglog("Scroll event: offset now:", sn.scrollTop,
                  "_lastSetScroll:", this._lastSetScroll);
 
+        // Set the node and notify subscribers of the StickContainer
+        // By extending StickyContainer, we can set the scroll node to be that of the
+        // ScrolPanel to allow any `<Sticky>` children to be sticky, namely DateSeparators.
         this.node = sn;
+        // Update subscribers - arbitrarily nested `<Sticky>` children
         this.notifySubscribers(ev);
 
         // Sometimes we see attempts to write to scrollTop essentially being