The current implementation of

jQuery.event.fix()

uses

!event.which

to determine whether to alter its value, which causes it to get mangled if its value is 0:

// Add which for key events
if ( !event.which && ((event.charCode || event.charCode === 0) ? event.charCode : event.keyCode) )
	event.which = event.charCode || event.keyCode;

This would be a non-issue except that checking whether

event.which === 0

is the only way to distinguish between certain keys in Opera (notoriously, the single-quote key and the right arrow key). Below is an implementation which corrects this problem by explicitly testing whether

event.which

et al. have been defined.

// Add which for key events
if ( typeof event.which == "undefined" && (typeof event.charCode != "undefined" || typeof event.keyCode != "undefined") )
	event.which = typeof event.charCode != "undefined" ? event.charCode : event.keyCode;

Attached are a simple test case and a diff of event.js against revision 4750.