For the next version I added the ability to hold Shift to drag anywhere inside the rectangle in order to move it. In order to educate about this, whenever the filter becomes current it shows a status message for 5 seconds:
Click in rectangle + hold Shift to drag, Wheel to zoom, or Ctrl+Wheel to rotate
(Inkscape does something like this I always found helpful.)
Also, you previously requested that the positioning lock function be remembered for the app session, and I will be adding that since I developed a technique for doing that for the rich text editor’s toolbar collapse state.