>100% what :-}
>
>FWIW, didn't see the behaviour on Chrome and IE stopped doing it. Almost certainly some temporary quirk at this end.....

There is an option to increase the view size of the browser. In IE, this is in File\View\Zoom 100%. If this is changed, this may create such an effect. If that would have been something related to that, it would have explained why you didn't get it with other browsers.