Determine most recent highlighted annotation by its updated date

src/sidebar/components/ThreadList.tsx

@@ -187,6 +187,7 @@ export default function ThreadList({ threads }: ThreadListProps) {
   const store = useSidebarStore();
   const editing = store.countDrafts() > 0;
   const highlightedAnnotations = store.highlightedAnnotations();
+  const allAnnotations = store.allAnnotations();
 
   // Get the `$tag` of the most recently created unsaved annotation.
   const newAnnotationTag = (() => {
@@ -211,20 +212,32 @@ export default function ThreadList({ threads }: ThreadListProps) {
     }
   }, [store, newAnnotationTag]);
 
-  const mostRecentlyHighlightedAnnotationId = useMemo(
-    // If multiple highlighted annotations exist, assume that the last one in
-    // the list is the most recent.
-    () => highlightedAnnotations[highlightedAnnotations.length - 1],
-    [highlightedAnnotations],
-  );
+  const mostRecentHighlightedAnnotationId = useMemo(() => {
+    const highlightedAnnos = allAnnotations.filter(
+      anno => anno.id && highlightedAnnotations.includes(anno.id),
+    );
+    // Get the annotation with most recent updated field, which contains a
+    // date in ISO format. This means their alphabetical and chronological
+    // orders match.
+    const mostRecentHighlightedAnnotation =
+      highlightedAnnos.reduce<Annotation | null>(
+        (mostRecent, current) =>
+          !mostRecent || mostRecent.updated < current.updated
+            ? current
+            : mostRecent,
+        null,
+      );
+
+    return mostRecentHighlightedAnnotation?.id;
+  }, [allAnnotations, highlightedAnnotations]);
 
   // Scroll to the most recently highlighted annotation, unless creating/editing
   // another annotation
   useEffect(() => {
-    if (!editing && mostRecentlyHighlightedAnnotationId) {
-      setScrollToId(mostRecentlyHighlightedAnnotationId);
+    if (!editing && mostRecentHighlightedAnnotationId) {
+      setScrollToId(mostRecentHighlightedAnnotationId);
     }
-  }, [editing, mostRecentlyHighlightedAnnotationId]);
+  }, [editing, mostRecentHighlightedAnnotationId]);
 
   // Effect to scroll a particular thread into view. This is mainly used to
   // scroll a newly created annotation into view.

src/sidebar/components/test/ThreadList-test.js

@@ -57,6 +57,7 @@ describe('ThreadList', () => {
       unsavedAnnotations: sinon.stub().returns([]),
       countDrafts: sinon.stub().returns(0),
       highlightedAnnotations: sinon.stub().returns([]),
+      allAnnotations: sinon.stub().returns([]),
     };
 
     fakeTopThread = {
@@ -193,13 +194,65 @@ describe('ThreadList', () => {
       assert.notCalled(fakeScrollTop);
     });
 
-    it('should set the scroll container `scrollTop` to first highlighted annotation', () => {
-      fakeStore.highlightedAnnotations.returns(['t2', 't3']);
-      createComponent();
+    [
+      {
+        annotationUpdates: {
+          t2: '2024-01-01T10:40:00',
+          t3: '2024-01-01T10:41:00', // Most recent
+          t4: '2024-01-01T10:39:00',
+        },
+        // The most recent highlighted annotation is the third.
+        // At default height (200) should be at 400px.
+        expectedScrollTop: 400,
+      },
+      {
+        annotationUpdates: {
+          t1: '2024-01-01T10:42:00', // Most recent
+          t3: '2024-01-01T10:39:00',
+          t4: '2024-01-01T10:39:00',
+        },
+        // The most recent highlighted annotation is the first.
+        // At default height (200) should be at 0px.
+        expectedScrollTop: 0,
+      },
+      {
+        annotationUpdates: {
+          t1: '2024-01-01T10:42:00',
+          t3: '2024-01-01T10:39:00',
+          t4: '2024-01-01T10:51:00', // Most recent
+        },
+        // The most recent highlighted annotation is the fourth.
+        // At default height (200) should be at 600px.
+        expectedScrollTop: 600,
+      },
+      {
+        annotationUpdates: {
+          t1: '2024-01-01T10:42:00',
+          t3: '2024-01-01T10:39:00',
+          t2: '2024-01-01T10:51:00', // Most recent
+        },
+        // The most recent highlighted annotation is the second.
+        // At default height (200) should be at 200px.
+        expectedScrollTop: 200,
+      },
+    ].forEach(({ annotationUpdates, expectedScrollTop }) => {
+      it('should set the scroll container `scrollTop` to most recent highlighted annotation', () => {
+        fakeStore.highlightedAnnotations.returns(
+          Object.keys(annotationUpdates),
+        );
+        fakeStore.allAnnotations.returns([
+          {}, // Discarded
+          { id: 't1', updated: annotationUpdates.t1 },
+          { id: 't2', updated: annotationUpdates.t2 },
+          { id: 't3', updated: annotationUpdates.t3 },
+          { id: 't4', updated: annotationUpdates.t4 },
+          { id: 't5', updated: annotationUpdates.t5 },
+        ]);
+
+        createComponent();
 
-      // The last highlighted annotation is the third in the collection of
-      // threads. At default height (200) should be at 400px.
-      assert.calledWith(fakeScrollTop, 400);
+        assert.calledWith(fakeScrollTop, expectedScrollTop);
+      });
     });
   });