The Finder is busy while Xojo running project is in the debugger

Current Xojo/macOS Tahoe:

I was debugging a project when I realized the code will try to load a banner… and no banner - yet - in images folder.

So, I let Xojo in the debugger and go to move/copy the correct image banner into images.
I was unable to do that in the Finder because I have the spinning rainbox cursor when I try to do that.
So, I killed the debugging session (stop in the black round button), go back to the Finder and was able to move/copy the Banner image file.

Just curious: what happens when you do nothing in the debugger ? (or what Xojo is doing in my back to suspend the multitasking Finder / macOS ?)

Your observation is correct and is related to file system locking and the way macOS system calls work. The simplest solution, as you did, is to close the debugger.