Bug 1766560: Don't ignore the first line start of a RemoteAccessible when retrieving the next line. r=morgan

The code previously returned failure for array index 0, but it did this for both movement directions.
It should only do this for eDirPrevious, since we decrement index in that case.

Differential Revision: https://phabricator.services.mozilla.com/D144785

accessible/base/TextLeafRange.cpp

@@ -523,7 +523,8 @@ TextLeafPoint TextLeafPoint::FindLineStartSameRemoteAcc(
     }
   }
   MOZ_ASSERT(index <= lines->Length());
-  if ((aDirection == eDirNext && index == lines->Length()) || index == 0) {
+  if ((aDirection == eDirNext && index == lines->Length()) ||
+      (aDirection == eDirPrevious && index == 0)) {
     return TextLeafPoint();
   }
   // index points at the line start after mOffset.

accessible/tests/browser/e10s/browser_caching_text.js

@@ -26,6 +26,7 @@ ef gh</pre>
 <p id="linksStartEnd"><a href="https://example.com/">a</a>b<a href="https://example.com/">c</a></p>
 <p id="linksBreaking">a<a href="https://example.com/">b<br>c</a>d</p>
 <p id="p">a<br role="presentation">b</p>
+<p id="leafThenWrap" style="width: 2ch; word-break: break-word;"><span>a</span>bc</p>
   `,
   async function(browser, docAcc) {
     for (const id of ["br", "pre"]) {
@@ -157,6 +158,11 @@ ef gh</pre>
       [1, 2, "b", 1, 2],
     ]);
     testTextAtOffset(p, BOUNDARY_PARAGRAPH, [[0, 2, "ab", 0, 2]]);
+    const leafThenWrap = findAccessibleChildByID(docAcc, "leafThenWrap");
+    testTextAtOffset(leafThenWrap, BOUNDARY_LINE_START, [
+      [0, 1, "ab", 0, 2],
+      [2, 3, "c", 2, 3],
+    ]);
   },
   { chrome: true, topLevel: true, iframe: true, remoteIframe: true }
 );