Keeping Renoise up front is called “Modal”. Disabling Resize can be done by changing the window type of the frame and keeping it on a specific spot could be done by removing the frame completely.
But for what purpose would you like it to have a fixed position?
when running renoise together with other applications (rewire) i’m adjusting renoise so that it fits nicely into a specific area of the other apps (and change renoise theme so that it blends into the environment)
if renoise could be in that fixed position and modal, it would make things much easier in the workflow