I just stumbled upon a bug in my software that happened only on Firefox. The reason was that the event didn't have offsetX and offsetY defined.
I managed to fix it thanks to this.
Anyways, my question is not a programming help request. I'm just curious why these properties are undefined in Firefox? What is the reason behind it?
I did look through: DOM3 UIEvent Spec., DOM3 MouseEvent Spec. and DOM2 MouseEvent Spec..
It appears that neither of the properties are mentioned there, so, I suppose that's an unofficial property introduced in other browsers?
Although mentioned in the W3 specification, the offsetx/offsety properties themselves are implemented inconsistently across browsers.
While supported in IE, Webkit browsers and Opera, they all function slightly different to the specifications requirements, except for IE - according to the "Calculating offsetX, offsetY" section here.
The properties aren't supported in Firefox at all - it appears to be a long-time bug that is yet to be resolved.
"I suppose that's an unofficial property introduced in other browsers?"
I believe it's an official property, that just hasn't been implemented in Firefox. If it was a non-official IE property, it wouldn't have been implemented in the Webkit/Opera browsers, mentioned in the W3 spec nor would Firefox actually be trying to implement it (check out the bug link above).