Xubuntu 17.04: Problem with radical change in middle mouse button behavior

[Jeffery Small]

I've posted to various ubuntu/xubuntu/xfce forums regarding this, with no
responses, so I'm hoping that someone on the development team can help with
this problem.

I've got a very serious problem with the middle mouse button behavior since
upgrading from Xubuntu 16.10 to 17.04 which is seriously interfering with
my daily operations.

Ever since the upgrade I've lost my old control of the use of the middle
button. For example, I used to be able to use it for gesture commands in
my CAD program running under Windows-10 in a QEMU/KVM window as well as in
the Firefox browser using the All-in-One Gestures Add-on. Everything worked
fine for years until 17.04 where the middle mouse press/hold is being
preempted as a pseudo-scroll function. As it affects everything from the
browser to programs running under KVM, I'm pretty sure this is something
new in either the kernel drivers or xfce4.

I have a standard 3-button Sun USB mouse with no scroll-wheel, on a desktop
system with no touchpad. I desperately need to get back my old behavior,
but I don't know what has changed and I cannot find any documentation for
this or any controls in the Settings menus for this sort of mouse behavior.
I need to deactivate this scrolling behavior and restore application-level
control to the mouse button.

I'll provide additional info if needed.

Thanks for any guidance.
--
Jeffery Small