Add a unit-test to check that the sortByVisibility parameter, in getVisibleElements, works correctly

test/unit/ui_utils_spec.js

@@ -690,6 +690,34 @@ describe('ui_utils', function() {
       scrollOverDocument(pages, true);
     });
 
+    it('handles `sortByVisibility` correctly', function() {
+      const scrollEl = {
+        scrollTop: 75,
+        scrollLeft: 0,
+        clientHeight: 750,
+        clientWidth: 1500,
+      };
+      const views = makePages([
+        [[100, 150]],
+        [[100, 150]],
+        [[100, 150]],
+      ]);
+
+      const visible = getVisibleElements(scrollEl, views);
+      const visibleSorted = getVisibleElements(scrollEl, views,
+                                               /* sortByVisibility = */ true);
+
+      const viewsOrder = [], viewsSortedOrder = [];
+      for (const view of visible.views) {
+        viewsOrder.push(view.id);
+      }
+      for (const view of visibleSorted.views) {
+        viewsSortedOrder.push(view.id);
+      }
+      expect(viewsOrder).toEqual([0, 1, 2]);
+      expect(viewsSortedOrder).toEqual([1, 2, 0]);
+    });
+
     it('handles views being empty', function() {
       const scrollEl = {
         scrollTop: 10,