Fix negative offset in Select-to-Speak that causes issues when resuming

When resuming from the end of a nodeGroupItem,
findNodeFromNodeGroupByCharIndex returns a node with a -1 offset. This
will throw exceptions. This CL fixes this bug.

Bug: 1167260
Change-Id: I2fe466a07f3daa2103dec53e6ebcedd5be27d285
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638863
Commit-Queue: Lei Shi <leileilei@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845406}

chrome/browser/resources/chromeos/accessibility/select_to_speak/paragraph_utils.js

@@ -505,9 +505,18 @@ export class ParagraphUtils {
       // We iterate over each nodeGroupItem until the |charIndex| is pointing
       // before the current node's name.
       if (currentNodeGroupItem.startChar + currentNodeNameLength > charIndex) {
-        // The currentNodeOffset is the position of the |charIndex| within the
-        // current nodeGroupItem.
-        const currentNodeOffset = charIndex - currentNodeGroupItem.startChar;
+        // The |currentNodeOffset| is the position of the |charIndex| within the
+        // current nodeGroupItem. ParagraphUtils.getNodeName returns a string
+        // without an ending space character, though |charIndex| is based on a
+        // string that was generated with an extra space character between each
+        // nodeGroupItems. Thus, there might be a gap between the previous
+        // nodeGroupItem and the current nodeGroupItem. If
+        // |currentNodeGroupItem.startChar| is greater than |charIndex|, that
+        // means the |charIndex| is pointing to the gap. In such case, we set
+        // |currentNodeOffset| to 0 and return the beginning of the current
+        // nodeGroupItem.
+        const currentNodeOffset =
+            Math.max(0, charIndex - currentNodeGroupItem.startChar);
         if (!currentNodeGroupItem.hasInlineText) {
           // If the nodeGroupItem does not have inline text, we return the
           // corresponding node and the current offset.

chrome/browser/resources/chromeos/accessibility/select_to_speak/paragraph_utils_unittest.js

@@ -567,6 +567,17 @@ SYNC_TEST_F(
       assertEquals(result.node.name, fourthInline);
       assertEquals(result.offset, 3);
 
+      // Pointing to the gap between the fourthInline and thirdStatic.
+      result = ParagraphUtils.findNodeFromNodeGroupByCharIndex(
+          nodeGroup, 50 /* charIndex */);
+      assertEquals(result.node.name, thirdStatic);
+      assertEquals(result.offset, 0);
+
+      result = ParagraphUtils.findNodeFromNodeGroupByCharIndex(
+          nodeGroup, 51 /* charIndex */);
+      assertEquals(result.node.name, thirdStatic);
+      assertEquals(result.offset, 0);
+
       result = ParagraphUtils.findNodeFromNodeGroupByCharIndex(
           nodeGroup, 52 /* charIndex */);
       assertEquals(result.node.name, thirdStatic);

chrome/browser/resources/chromeos/accessibility/select_to_speak/select_to_speak_navigation_control_test.js

@@ -433,6 +433,41 @@ TEST_F(
           });
     });
 
+TEST_F(
+    'SelectToSpeakNavigationControlTest', 'PauseResumeAtTheEndOfNodeGroupItem',
+    function() {
+      const bodyHtml = `
+        <p id="p1">Sentence <span>one</span>. Sentence two.</p>
+      `;
+      this.runWithLoadedTree(
+          this.generateHtmlWithSelectedElement('p1', bodyHtml), () => {
+            this.triggerReadSelectedText();
+
+            // Finishes the second word.
+            this.mockTts.speakUntilCharIndex(13);
+            assertTrue(this.mockTts.currentlySpeaking());
+            assertEquals(this.mockTts.pendingUtterances().length, 1);
+            this.assertEqualsCollapseWhitespace(
+                this.mockTts.pendingUtterances()[0],
+                'Sentence one . Sentence two.');
+
+            // Hitting pause will stop the current TTS.
+            selectToSpeak.onSelectToSpeakPanelAction_(
+                chrome.accessibilityPrivate.SelectToSpeakPanelAction.PAUSE);
+            assertFalse(this.mockTts.currentlySpeaking());
+            assertEquals(this.mockTts.pendingUtterances().length, 0);
+
+            // Hitting resume will start from the remaining content of the
+            // paragraph.
+            selectToSpeak.onSelectToSpeakPanelAction_(
+                chrome.accessibilityPrivate.SelectToSpeakPanelAction.RESUME);
+            assertTrue(this.mockTts.currentlySpeaking());
+            assertEquals(this.mockTts.pendingUtterances().length, 1);
+            this.assertEqualsCollapseWhitespace(
+                this.mockTts.pendingUtterances()[0], '. Sentence two.');
+          });
+    });
+
 TEST_F(
     'SelectToSpeakNavigationControlTest', 'PauseResumeFromKeystrokeSelection',
     function() {