I was able get to a Restore Point created a couple of days ago and that problem is solved. I will try to get back to your problem on Monday.
It looks like I discovered a serious security problem in Windows.
At work our machines have a program called Deep Freeze that returns the computer as set up by IT each time it is rebooted, so I can safely test things like this.
Turns out if you save even a MS Word *.doc file (can be blank) to the desktop and then rename to *.stl it causes Windows Explorer to go into a failure loop.
Only discovered this because I thought you renamed an stl in order to attach it here since you can't attach stl directly.
Getting back to the original problem, Fusion is not the tool you want it to be. As posted earlier, Fusion is a solid modeller, Blender is not. Blender is a mesh modeller - so if you want "like for like" comparisons, download 3ds max. That will give you all the push/pull mesh modelling tools you can imagine (and a few more). It supports STL files too (unlike Windows explorer apparently!)