Absolutely eliminate "Automatic new Desktop when dragging" As far as I know I do have only one (1) Desktop enabled
I'm looking at this web page right now in Safari.
Safari is a normal window (say, 50% of the width and height of the physical screen, and it is sitting right in the middle of the physical screen).
This is a mid 2015 mbp with no external monitors attached.  Every piece of software is the latest as of writing
Using the pointer, I grab an image (say, one of the small icons on this screen).
I grag it to the extreme right of the desktop - as if I am going to drop it on the desktop near the right
But I keep dragging it, and hold it on the extreme right edge of the screen


IN FACT WHAT HAPPENS IS......


After a few moments, the "whole desktop" will slide to a new desktop on the "right".
Strangely, this new desktop has a different color background to my usual desktop
The "new" Desktop appears to be identical to my usual only desktop (same icons scattered around etc), except only the background color is different


And click here for a video !!!

http://tinypic.com/r/jq2nis/9

I want to completely disable this "sliding" function.  (Or at least know what the hell it is!!) Is it possible?

Related!  How do I correctly set strictly ONLY ONE desktop?
Not seeing this behavior. I have several desktops enabled, yes macOS will happily slide to the next available if I drag a file/window to the extreme right of the screen, but it won't create NEW desktops. Once I hit #4 (the last desktop I have enabled) holding a file against the right side of the screen doesn't trigger any action. Perhaps you have more than 1 screen enabled in Mission Control? Remove them, it should not auto-create more. (added as answer per request!)