Try this demo http://jquerytools.org/demos/overlay/external.html in IE9 - mask width causes horizontal scroll to appear - https://dl.dropbox.com/u/171648/jQueryTools1.2.7_ExposeMask_IE9_bug.png

IE8 is OK.

The bug in toolbox.expose.js seems to be in the "the horror case" condition which is applied to all versions of IE ($.browser.msie), but IE9 is quite "well behaving" browser :-), so should be excluded from case.

May be try to limit "the horror case" to IE8 and lower by adding $.support.opacity == false condition, which will not be applied to IE9+ (but I didn't try this fix yet).