Bug 1782563 - Support END_OF_TEXT offset in TextLeafPoint constructor. r=Jamie

Differential Revision: https://phabricator.services.mozilla.com/D153386

accessible/base/TextLeafRange.cpp

@@ -439,14 +439,24 @@ TextLeafPoint::TextLeafPoint(Accessible* aAcc, int32_t aOffset) {
   if (aOffset != nsIAccessibleText::TEXT_OFFSET_CARET && aAcc->HasChildren()) {
     // Find a leaf. This might not necessarily be a TextLeafAccessible; it
     // could be an empty container.
-    for (Accessible* acc = aAcc->FirstChild(); acc; acc = acc->FirstChild()) {
+    auto GetChild = [&aOffset](Accessible* acc) -> Accessible* {
+      return aOffset != nsIAccessibleText::TEXT_OFFSET_END_OF_TEXT
+                 ? acc->FirstChild()
+                 : acc->LastChild();
+    };
+
+    for (Accessible* acc = GetChild(aAcc); acc; acc = GetChild(acc)) {
       mAcc = acc;
     }
-    mOffset = 0;
+    mOffset = aOffset != nsIAccessibleText::TEXT_OFFSET_END_OF_TEXT
+                  ? 0
+                  : nsAccUtils::TextLength(mAcc);
     return;
   }
   mAcc = aAcc;
-  mOffset = aOffset;
+  mOffset = aOffset != nsIAccessibleText::TEXT_OFFSET_END_OF_TEXT
+                ? aOffset
+                : nsAccUtils::TextLength(mAcc);
 }
 
 bool TextLeafPoint::operator<(const TextLeafPoint& aPoint) const {