Ensure empty strings in Output do not reset queueMode

Change-Id: Ic0a737a7dc4663761f8596a82a3675b52bdfa48d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096913
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749475}

chrome/browser/resources/chromeos/accessibility/chromevox/background/background_test.js

@@ -2483,3 +2483,20 @@ TEST_F('ChromeVoxBackgroundTest', 'ReadWindowTitle', function() {
             .replay();
       });
 });
+
+TEST_F('ChromeVoxBackgroundTest', 'OutputEmptyQueueMode', function() {
+  const mockFeedback = this.createMockFeedback();
+  this.runWithLoadedTree('<p>unused</p>', function(root) {
+    const output = new Output();
+    Output.forceModeForNextSpeechUtterance(QueueMode.CATEGORY_FLUSH);
+    output.append_(
+        output.speechBuffer_, new Spannable(''),
+        {annotation: [new Output.Action()]});
+    output.withString('test');
+    mockFeedback.clearPendingOutput()
+        .call(output.go.bind(output))
+        .expectSpeechWithQueueMode('', QueueMode.CATEGORY_FLUSH)
+        .expectSpeechWithQueueMode('test', QueueMode.CATEGORY_FLUSH)
+        .replay();
+  });
+});

chrome/browser/resources/chromeos/accessibility/chromevox/background/output.js

@@ -515,7 +515,13 @@ Output = class {
       }
 
       ChromeVox.tts.speak(buff.toString(), queueMode, speechProps);
-      queueMode = QueueMode.QUEUE;
+
+      // Skip resetting |queueMode| if the |text| is empty. If we don't do this,
+      // and the tts engine doesn't generate a callback, we might not properly
+      // flush.
+      if (text !== '') {
+        queueMode = QueueMode.QUEUE;
+      }
     }
     if (this.speechRulesStr_.str) {
       LogStore.getInstance().writeTextLog(