Fixed the offset calculation in when scrolling

Therefore added a new function getAncestorOffset()

extension/content/firebug/lib/dom.js

@@ -437,6 +437,11 @@ Dom.findPrevious = function(node, criteria, downOnly, maxRoot)
 // ********************************************************************************************* //
 // Graphics
 
+/**
+ * Gets the absolute offset of an element
+ * @param {Element} elt Element to get the info for
+ * @returns {Object} x and y offset of the element
+ */
 Dom.getClientOffset = function(elt)
 {
     function addOffset(elt, coords, view)
@@ -505,6 +510,25 @@ Dom.getLTRBWH = function(elt)
     return dims;
 };
 
+/**
+ * Gets the offset of an element relative to an ancestor
+ * @param {Element} elt Element to get the info for
+ * @param {Element} ancestor Ancestor element used as origin
+ */
+Dom.getAncestorOffset = function(elt, ancestor)
+{
+    var offset = {x: 0, y: 0};
+    var offsetParent = elt;
+    do
+    {
+        offset.x += offsetParent.offsetLeft;
+        offset.y += offsetParent.offsetTop;
+        offsetParent = offsetParent.offsetParent;
+    } while (offsetParent && offsetParent !== ancestor);
+
+    return offset;
+};
+
 /**
  * Gets the offset size of an element
  * @param {Object} elt Element to move
@@ -649,10 +673,7 @@ Dom.scrollTo = function(element, scrollBox, alignmentX, alignmentY, scrollWhenVi
     if (!scrollBox)
         return;
 
-    var offset = {
-        x: element.offsetLeft,
-        y: element.offsetTop
-    };
+    var offset = Dom.getAncestorOffset(element, scrollBox);
 
     if (!alignmentX)
         alignmentX = "centerOrLeft";