keyboard-selection-extension fix

AppKit/CPTextView/CPTextView.j

@@ -545,6 +545,7 @@ var kDelegateRespondsTo_textShouldBeginEditing
     }
 
     [self _setSelectedRange:CPMakeRange(_selectionRange.location + [string length], 0) affinity:0 stillSelecting:NO overwriteTypingAttributes:NO];
+    _startTrackingLocation = _selectionRange.location;
 
     [self didChangeText];
     [_layoutManager _validateLayoutAndGlyphs];