I have gentoo and xfce 4.12. My application menu, when i hover it: http://mpc.iso.vc/screenshot.png What i need do, to disable alpha on it?
You need to adjust the opacity of popup windows.
The following instructions are applicable to Xfce 4.8 and newer.
Open Settings Manager or run
Go to Window Manager Tweaks - Compositor
Look for the option Opacity of popup windows: (toward last entry)
Drag the slider control until the furthest right (toward Opaque)
Finally, click Close
Now the popup menu is no longer transparent and fully opaque.
After: Fully opaque