I'm using Xubuntu 14.04 with xubuntu-dev/xfce-4.12 ppa enable.
The transparency of the windows can not be set using Alt+mouse-roll (its work on previous version 4.11.2).
Nevertheless it still works by using the pad with the horizontal roll.
The shortcut may change ?
NB : I change the shortcut to move windows/zoom (Alt initially, Hyper now).
It's using the same modifier as move/resize ("alt" by default) - This option is not compatible with zoom (as using the same modifier), disable zoom and the transparency shortcut will work again.
Alternatively, you can use horizontal scrolling to adjust transparency.
Yes I change the zoom shortcut but the transparency does not work and I have no horizontal scroll on my mouse.
The option worked on last version of xfwm4 but since the update it does not.
Oh sorry, it works !
I disable the zoom in the parameters editor and the transparency work with mouse-scroll.
The main difference with the version 4.11.2 is that the shortcut for tranparency is set by the accessibility shortcut to move/resize windows now. In the previous version, the transparency shortcut stay Alt whatever the move/resize shortcut is.