##
win.setExecAfterAllReturn()

    function f(w) {
        // w takes the value of w1 or w2, according to which
        // window called the function after all of its serverFrames returned
    }
    w1.setExecAfterAllReturn(f)
    w2.setExecAfterAllReturn(f)

This method is called after all WebServer requests from serverFrames within the window have returned. By default, the "hideLed()" method of that window is called, i.e. the leftmost button turns grey. Note that when you redefine this function, the button won't turn grey any more, unless you specify this explicitly in the function.