Populate aria-owns attribute on iron-list + test

iron-list.js

@@ -1323,10 +1323,17 @@ Polymer({
         }
       });
     } else {
+      let order = [];
       this._iterateItems(function(pidx, vidx) {
         this.translate3d(0, y + 'px', 0, this._physicalItems[pidx]);
         y += this._physicalSizes[pidx];
+        if (this._physicalItems[pidx].id) {
+          order.push(this._physicalItems[pidx].id);
+        }
       });
+      if (order.length) {
+        this.setAttribute('aria-owns', order.join(' '));
+      }
     }
   },
 

test/basic.html

@@ -20,7 +20,7 @@
 
   <test-fixture id="trivialList">
     <template>
-      <x-list></x-list>
+      <x-list id$="tl[[index]]"></x-list>
     </template>
   </test-fixture>
 
@@ -145,6 +145,38 @@
       assert.equal(list.firstVisibleIndex, 99);
     });
 
+    test('expected aria-owns while scrolling', function() {
+      list.items = buildDataSet(100);
+      PolymerFlush();
+
+      const getExpectedAriaOwns = () => {
+        return
+            // Physical children.
+            Array.from(list.querySelectorAll('x-list'))
+
+            // Skip selected node.
+            .filter(child => child.getBoundingClientRect().top >= -10000)
+
+            // Sort by |top|.
+            .sort((left, right) => left.getBoundingClientRect().top - right.getBoundingClientRect().top)
+
+            // Get IDs.
+            .map(child => child.id)
+
+            // Create expected |aria-owned|.
+            .join(' ');
+      };
+
+      // Make sure we're virtualizing nodes.
+      assert.isTrue(Array.from(list.querySelectorAll('x-list')).length < list.items.length);
+
+      // Scroll by 7 because it's not a 'nice round number'.
+      for (let i = 0; i < list.items.length; i += 7) {
+        list.scrollToIndex(i);
+        assert.equal(list.getAttribute('aria-owns'), getExpectedAriaOwns());
+      }
+    });
+
     test('scroll to index while not attached', function() {
       var tmpList = document.createElement('iron-list');
       dom(tmpList).appendChild(document.createElement('template'));