I use this function (var newwin = editor.ShowDialog(null, mURL, editor, mSize);)  to open a custom dialog window. 

In dialog form, I use function as bellow to get text selection from editor control. It works fine. 

But in IE 11, argument "window" is not a object. So I cannot get selected text in editor.

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var editor = Window_GetDialogArguments(window); 

 function Window_GetDialogArguments(win) {

                    var top = win.top;

                    try {

                        var opener = top.opener;

                        if (opener && opener.document._dialog_arguments)

                            return opener.document._dialog_arguments;

                    }

                    catch (x) {

                    }

                    if (top.document._dialog_arguments)

                        return top.document._dialog_arguments;

                    if (top.dialogArguments)

                        return top.dialogArguments;

                    return top.document._dialog_arguments;

   }