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.