KB2846071 Breaks clientX and clientY

Since installing KB2846071 a large number of users of our very mature web application are having problems. The application was working fine before the patch was installed, and works again after the patch has been uninstalled.

This is impacting thousands of users.

I've tested a number of workarounds, with limited success.

I have found the problem is quite specific to how the event is fired.

Given the function

function showPos() {

    alert(event.clientX);

}

And HTML

     <INPUT id="imok" onclick="showPos();" />

will display a valid mouse coordinate

      <INPUT id="imnot" onfocus="showPos();" />

does not. It will display a negative number, related to the window position on the screen (looks to be position * -1). It is completely unrelated to the mouse coordinates.