By default, Xfce has a “grab key” that you can use to, well, grab a window and move it around. By default, it is the ALT. Which can be a problem with certain apps, like Inkscape, to access some of their tools or options.

You can quickly change the grab key — or turn it off, if you don’t need it all.

Go to Settings→Window Manager Tweaks. Open the Accessibility tab and change the Key used to grab and move windows to what you see fit. To turn it off, select None.

XFCE’s accessibility settings