Bug 1675172 - Prevent an infinite loop when finding the line end boundary for an offset of 0, r=Jamie

Use a safer looping method, walking forward from aOffset to tmpOffset to prevent wrapping around at 0.

Differential Revision: https://phabricator.services.mozilla.com/D95826

accessible/generic/HyperTextAccessible.cpp

@@ -819,14 +819,15 @@ uint32_t HyperTextAccessible::FindLineBoundary(
       // layout could not find the offset for us. This can happen with certain
       // inline-block elements.
       if (nextLineBeginOffset <= aOffset) {
-        // Walk back from the tmpOffset to the offset we started from,
+        // Walk forward from the offset we started from up to tmpOffset,
         // stopping after a line end character.
-        nextLineBeginOffset = tmpOffset;
-        while (nextLineBeginOffset >= aOffset &&
-               !IsLineEndCharAt(nextLineBeginOffset - 1)) {
-          nextLineBeginOffset--;
+        nextLineBeginOffset = aOffset;
+        while (nextLineBeginOffset < tmpOffset) {
+          if (IsLineEndCharAt(nextLineBeginOffset)) {
+            return nextLineBeginOffset + 1;
+          }
+          nextLineBeginOffset++;
         }
-        return nextLineBeginOffset;
       }
 
       return nextLineBeginOffset;