If you have enabled the Menu Bar, you cannot grab the window at the top anywhere in line with the menu bar frame. You have to go below it to move the window. Considering every other program or browser, you grab the top of the window, not 1/3 of an inch down, this is unorthodox and a PITA. Please correct the issue with Walnut as I'd like to keep using it.
Edit: I just noticed, if you have multiple tabs open and not just one, you really can't grab the window ANY place because the tabs are in the way. You can only move it if you have one or two tabs open and click in the empty area to the right of those tabs. Multiple tabs put you to clicking in line with the menu frame, and that is a no go period. So I can't use this for now until you correct this problem.