In xfwm4-settings I have both "Snap windows to screen border" and "Snap windows to other windows" enabled. As a result, when I move a window around, in addition to snapping to the screen edge and to other windows, it will also snap to the top edge of the Xfce panel.
However, when I'm resizing a window, the "snap to panel" functionality doesn't work, and I have to manually stop resizing the window at approximately the pixel where the panel begins. It would be nice if the resize snapped to the top of the panel too, so I don't need to do it manually. (IIRC, KDE window resize works this way.)
See screenshots for details:
There is no snapping on resize in 4.6
git head has preliminary support for snapping on resize. But the panel resides on a upper layer so chances are that it'll be ignored.
I tested with latest git revision and it works fine with the panel too.