Support editors in fixed-position elements

This might very well break something else, but passes my minimal tests.

Author: marijnh

lib/codemirror.js

@@ -1106,13 +1106,13 @@ var CodeMirror = (function() {
     function paddingLeft() {return lineSpace.offsetLeft;}
 
     function posFromMouse(e, liberal) {
-      var offW = eltOffset(wrapper), x = e.pageX(), y = e.pageY();
+      var offW = eltOffset(wrapper, true), x = e.e.clientX, y = e.e.clientY;
       // This is a mess of a heuristic to try and determine whether a
       // scroll-bar was clicked or not, and to return null if one was
       // (and !liberal).
       if (!liberal && (x - offW.left > wrapper.clientWidth || y - offW.top > wrapper.clientHeight))
         return null;
-      var offL = eltOffset(lineSpace);
+      var offL = eltOffset(lineSpace, true);
       var line = showingFrom + Math.floor((y - offL.top) / lineHeight());
       return clipPos({line: line, ch: charFromX(clipLine(line), x - offL.left)});
     }
@@ -1879,10 +1879,18 @@ var CodeMirror = (function() {
   }
 
   // Find the position of an element by following the offsetParent chain.
-  function eltOffset(node) {
-    var x = 0, y = 0;
-    for (var n = node; n; n = n.offsetParent) {x += n.offsetLeft; y += n.offsetTop;}
-    for (var n = node.parentNode; n != node.ownerDocument.body; n = n.parentNode) {x -= n.scrollLeft; y -= n.scrollTop;}
+  // If screen==true, it returns screen (rather than page) coordinates.
+  function eltOffset(node, screen) {
+    var doc = node.ownerDocument.body;
+    var x = 0, y = 0, hitDoc = false;
+    for (var n = node; n; n = n.offsetParent) {
+      x += n.offsetLeft; y += n.offsetTop;
+      // Fixed-position elements don't have the document in their offset chain
+      if (n == doc) hitDoc = true;
+    }
+    var e = screen && hitDoc ? null : doc;
+    for (var n = node.parentNode; n != e; n = n.parentNode)
+      if (n.scrollLeft != null) { x -= n.scrollLeft; y -= n.scrollTop;}
     return {left: x, top: y};
   }
   // Get a node's text content.