Fix off-by-one error by setting root thread visible:false

Ensure empty root threads are not counted when counting results
by always setting this empty thread `visible:false`

Fixes #3137

src/sidebar/helpers/build-thread.js

@@ -306,7 +306,11 @@ export default function buildThread(annotations, options) {
     thread.children = thread.children.filter(options.threadFilterFn);
   }
 
-  // Set visibility for threads
+  // Set visibility for threads.
+  // The root thread itself should be set as non-visible, to avoid confusion
+  // when counting visible threads. It's a container thread: its children
+  // are the top-level annotations.
+  thread.visible = false;
   thread = mapThread(thread, thread => {
     let threadIsVisible = thread.visible;
 

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

@@ -69,9 +69,9 @@ function createThread(fixture, options, keys) {
   return rootThread.children;
 }
 
-describe('sidebar/helpers/build-thread', function () {
-  describe('threading', function () {
-    it('arranges parents and children as a thread', function () {
+describe('sidebar/helpers/build-thread', () => {
+  describe('threading', () => {
+    it('arranges parents and children as a thread', () => {
       const thread = createThread(SIMPLE_FIXTURE);
       assert.deepEqual(thread, [
         {
@@ -378,6 +378,15 @@ describe('sidebar/helpers/build-thread', function () {
         ]);
       });
 
+      it('sets root thread visible:false so it does not get counted as a visible thread', () => {
+        // Always set the root-level thread `visible:false` such that it is
+        // not inadvertently counted as a visible thread. It doesn't contain
+        // any thread content itself; its children are top-level threads.
+        const thread = buildThread(SIMPLE_FIXTURE, defaultBuildThreadOpts);
+
+        assert.isFalse(thread.visible);
+      });
+
       it('shows threads containing replies that match the filter', () => {
         const threads = createThread(
           SIMPLE_FIXTURE,