Bug 1665662 - Ensure that at the end of a paragraph, after a line break, we return empty line offsets, r=Jamie

If in a textarea, a blank line is inserted, we need to return the offsets of just that new inner paragraph, or the braille display of a screen reader will show the previous line and not a blank one.

Differential Revision: https://phabricator.services.mozilla.com/D90655

accessible/generic/HyperTextAccessible.cpp

@@ -789,6 +789,14 @@ void HyperTextAccessible::TextAtOffset(int32_t aOffset,
         adjustedOffset = AdjustCaretOffset(adjustedOffset);
       }
 
+      if (IsEmptyLastLineOffset(adjustedOffset)) {
+        // We are on the last line of a paragraph where there is no text.
+        // For example, in a textarea where a new line has just been inserted.
+        // In this case, return offsets for an empty line without text content.
+        *aStartOffset = *aEndOffset = adjustedOffset;
+        break;
+      }
+
       if (IsLineEndCharAt(adjustedOffset)) {
         // Layout gives us different results for a paragraph with line breaks.
         // For the text, we get the text, for the line break, we get the text

accessible/tests/mochitest/text/test_paragraphboundary.html

@@ -52,6 +52,12 @@
       testTextAtOffset("li1", BOUNDARY_PARAGRAPH,
       [[0, 3, "• a", 0, 3]]);
 
+      // Test that a textarea has a blank paragraph at the end if it contains
+      // a line break as its last character.
+      testTextAtOffset("textarea_with_br", BOUNDARY_PARAGRAPH,
+      [[0, 15, "This is a test.\n", 0, 16],
+       [16, 16, "", 16, 16]]);
+
       SimpleTest.finish();
     }
 
@@ -81,5 +87,7 @@
   <p id="br_at_beginning"><br>a<br>b</p>
   <p id="pWithLink">a<a id="link" href="https://example.com/">bc</a>d</p>
   <ul id="ul"><li id="li1">a</li><li>b</li></ul>
+  <textarea id="textarea_with_br">This is a test.
+</textarea> <!-- This must be outdented for a correct test case -->
 </body>
 </html>