--- a/content/events/src/nsContentEventHandler.cpp
+++ b/content/events/src/nsContentEventHandler.cpp
@@ -210,18 +210,20 @@ static void AppendSubString(nsAString& a
 static PRUint32 CountNewlinesInXPLength(nsIContent* aContent,
                                         PRUint32 aXPLength)
 {
   NS_ASSERTION(aContent->IsNodeOfType(nsINode::eTEXT),
                "aContent is not a text node!");
   const nsTextFragment* text = aContent->GetText();
   if (!text)
     return 0;
-  NS_ASSERTION(aXPLength == PR_UINT32_MAX || aXPLength <= text->GetLength(),
-               "text offset is out-of-bounds");
+  // For automated tests, we should abort on debug build.
+  NS_ABORT_IF_FALSE(
+    (aXPLength == PR_UINT32_MAX || aXPLength <= text->GetLength()),
+    "aXPLength is out-of-bounds");
   const PRUint32 length = NS_MIN(aXPLength, text->GetLength());
   PRUint32 newlines = 0;
   for (PRUint32 i = 0; i < length; ++i) {
     if (text->CharAt(i) == '\n') {
       ++newlines;
     }
   }
   return newlines;
@@ -231,21 +233,27 @@ static PRUint32 CountNewlinesInNativeLen
                                             PRUint32 aNativeLength)
 {
   NS_ASSERTION(aContent->IsNodeOfType(nsINode::eTEXT),
                "aContent is not a text node!");
   const nsTextFragment* text = aContent->GetText();
   if (!text) {
     return 0;
   }
+  // For automated tests, we should abort on debug build.
+  NS_ABORT_IF_FALSE(
+    (aNativeLength == PR_UINT32_MAX || aNativeLength <= text->GetLength() * 2),
+    "aNativeLength is unexpected value");
   const PRUint32 xpLength = text->GetLength();
   PRUint32 newlines = 0;
   for (PRUint32 i = 0, nativeOffset = 0;
        i < xpLength && nativeOffset < aNativeLength;
        ++i, ++nativeOffset) {
+    // For automated tests, we should abort on debug build.
+    NS_ABORT_IF_FALSE(i < text->GetLength(), "i is out-of-bounds");
     if (text->CharAt(i) == '\n') {
       ++newlines;
       ++nativeOffset;
     }
   }
   return newlines;
 }
 #endif

--- a/widget/tests/window_composition_text_querycontent.xul
+++ b/widget/tests/window_composition_text_querycontent.xul
@@ -75,16 +75,18 @@ var iframe = document.getElementById("if
 var input = document.getElementById("input");
 var textareaInFrame;
 
 const nsIDOMWindowUtils = Components.interfaces.nsIDOMWindowUtils;
 
 const kIsWin = (navigator.platform.indexOf("Win") == 0);
 const kIsMac = (navigator.platform.indexOf("Mac") == 0);
 
+const kLFLen = kIsWin ? 2 : 1;
+
 function checkQueryContentResult(aResult, aMessage)
 {
   ok(aResult, aMessage + ": the result is null");
   if (!aResult) {
     return false;
   }
   ok(aResult.succeeded, aMessage + ": the query content failed");
   return aResult.succeeded;
@@ -1814,18 +1816,16 @@ function runCompositionEventTest()
 function runCharAtPointTest(aFocusedEditor, aTargetName)
 {
   aFocusedEditor.value = "This is a test of the\nContent Events";
                        // 012345678901234567890  12345678901234
                        // 0         1         2           3    
 
   aFocusedEditor.focus();
 
-  const kLFLen = kIsWin ? 2 : 1;
-
   const kNone = -1;
   const kTestingOffset   = [     0, 10,    20, 21 + kLFLen, 34 + kLFLen];
   const kLeftSideOffset  = [ kNone,  9,    19,       kNone, 33 + kLFLen];
   const kRightSideOffset = [     1, 11, kNone, 22 + kLFLen,       kNone];
 
   var editorRect = synthesizeQueryEditorRect();
   if (!checkQueryContentResult(editorRect,
         "runCharAtPointTest (" + aTargetName + "): editorRect")) {
@@ -1919,16 +1919,59 @@ function runCharAtPointAtOutsideTest()
                                        editorRect.top - 10);
   if (checkQueryContentResult(charAtPt,
         "runCharAtPointAtOutsideTest: charAtPt")) {
     ok(charAtPt.notFound,
        "runCharAtPointAtOutsideTest: charAtPt is found on outside of editor");
   }
 }
 
+function runBug722639Test()
+{
+  textarea.focus();
+  textarea.value = "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n";
+  textarea.value += textarea.value;
+  textarea.value += textarea.value; // 80 characters
+
+  var firstLine = synthesizeQueryTextRect(0, 1);
+  if (!checkQueryContentResult(firstLine,
+        "runBug722639Test: firstLine")) {
+    return;
+  }
+  var secondLine = synthesizeQueryTextRect(kLFLen, 1);
+  if (!checkQueryContentResult(secondLine,
+        "runBug722639Test: secondLine")) {
+    return;
+  }
+  var lineHeight = secondLine.top -  firstLine.top;
+  ok(lineHeight > 0,
+     "runBug722639Test: lineHeight must be positive");
+  is(secondLine.left, firstLine.left,
+     "runBug722639Test: the left value must be always same value");
+  var previousTop = secondLine.top;
+  for (var i = 2; i < textarea.value.length; i++) {
+    var currentLine = synthesizeQueryTextRect(kLFLen * i, 1);
+    if (!checkQueryContentResult(currentLine,
+           "runBug722639Test: " + i + "th currentLine")) {
+      return;
+    }
+    // NOTE: the top position may be 1px larger or smaller than other lines
+    //       due to sub pixel positioning.
+    if (Math.abs(currentLine.top - (previousTop + lineHeight)) <= 1) {
+      ok(true, "runBug722639Test: " + i + "th line's top is expected");
+    } else {
+      is(currentLine.top, previousTop + lineHeight,
+         "runBug722639Test: " + i + "th line's top is unexpected");
+    }
+    is(currentLine.left, firstLine.left,
+       "runBug722639Test: " + i + "th line's left is unexpected");
+    previousTop = currentLine.top;
+  }
+}
+
 function runTestOnAnotherContext(aPanelOrFrame, aFocusedEditor, aTestName)
 {
   aFocusedEditor.value = "";
 
   var editorRect = synthesizeQueryEditorRect();
   if (!checkQueryContentResult(editorRect, aTestName + ": editorRect")) {
     return;
   }
@@ -2403,16 +2446,17 @@ function runMaxLengthTest()
 
 function runTest()
 {
   runUndoRedoTest();
   runCompositionTest();
   runCompositionEventTest();
   runCharAtPointTest(textarea, "textarea in the document");
   runCharAtPointAtOutsideTest();
+  runBug722639Test();
   runFrameTest();
   runPanelTest();
   runMaxLengthTest();
 }
 
 ]]>
 </script>