Index: src/wp-admin/js/editor-expand.js
===================================================================
--- src/wp-admin/js/editor-expand.js	(revision 30813)
+++ src/wp-admin/js/editor-expand.js	(working copy)
@@ -937,13 +937,14 @@
 					} )
 					// Fade in when the mouse moves away form the editor area.
 					.on( 'mousemove.focus', function( event ) {
-						var nx = event.pageX,
-							ny = event.pageY;
+						var nx = event.clientX,
+							ny = event.clientY,
+							pageYOffset = window.pageYOffset;
 
 						if ( x && y && ( nx !== x || ny !== y ) ) {
 							if (
-								( ny <= y && ny < editorRect.top ) ||
-								( ny >= y && ny > editorRect.bottom ) ||
+								( ny <= y && ny < editorRect.top + pageYOffset ) ||
+								( ny >= y && ny > editorRect.bottom + pageYOffset ) ||
 								( nx <= x && nx < editorRect.left ) ||
 								( nx >= x && nx > editorRect.right )
 							) {
@@ -951,8 +952,8 @@
 								traveledY += Math.abs( y - ny );
 
 								if ( (
-									ny <= editorRect.top - buffer ||
-									ny >= editorRect.bottom + buffer ||
+									ny <= editorRect.top - buffer + pageYOffset ||
+									ny >= editorRect.bottom + buffer + pageYOffset ||
 									nx <= editorRect.left - buffer ||
 									nx >= editorRect.right + buffer
 								) && (