When you leftclick in IE any assigned click handlers will never have a "which" property on their event objects, despite having been through jQuery.event.fix().

The code in question is the following:

Add which for click: 1 == left; 2 == middle; 3 == right

Note: button is not normalized, so don't use it

if ( !event.which && event.button )

event.which = (event.button & 1 ? 1 : ( event.button & 2 ? 3 : ( event.button & 4 ? 2 : 0 ) ));

Since leftclicking sets the button property to 0 the above will never be triggered in IE, and IE will not get a which property on click events.