Assure all `thread` objects are assigned a unique `id`

src/sidebar/util/build-thread.js

@@ -27,7 +27,6 @@
  * Default state for new threads
  */
 const DEFAULT_THREAD_STATE = {
-  id: '__default__',
   collapsed: false,
   visible: true,
   replyCount: 0,
@@ -88,6 +87,7 @@ function setParent(threads, id, parents = []) {
     threads[parentId] = {
       ...DEFAULT_THREAD_STATE,
       children: [],
+      id: parentId,
     };
     // Link up this new thread to _its_ parent, which should be the original
     // thread's grandparent

src/sidebar/util/test/build-thread-test.js

@@ -184,11 +184,14 @@ describe('sidebar/util/build-thread', function () {
           references: ['3'],
         },
       ];
-      const thread = createThread(fixture);
+      // Get the threads with `id` key included
+      const thread = createThread(fixture, {}, ['id']);
+      // It should create a thread with an id of the missing annotation (3)
       assert.deepEqual(thread, [
         {
+          id: '3',
           annotation: undefined,
-          children: [{ annotation: fixture[0], children: [] }],
+          children: [{ id: '1', annotation: fixture[0], children: [] }],
         },
       ]);
     });