Apps default to full screen after Fall Update

Brink said:

The packages for the Store apps are stored in the "C:\Program Files\WindowsApps" protected folder.

Unfortunately, you won't be able to use the same properties trick on them though.

If you haven't already, you might see if resetting the affected apps may sort them.

Reset App in Windows 10 Apps Features Tutorials

The culprit was "Windows Defender notification icon" which actually appears twice in the Task Manager Start-up display despite the fact that I have de-activated Windows Defender.
I have disabled both entries and that has solved the problem.

Brink said:

Odd that would cause the apps to open full screen.

Indeed it is.

I think what may have been happening was that the position/size of the apps windows was not being saved.
In any event, I went through a binary search process of elimination; disabling non-windows services via config.msc and then when I established that none of those were the problem I went through the same process with the Start-up entries in Task Manager eventual narrowing it down to those two entries.

I will admit that my first reaction on seeing them in Task manager was to kick them out since I thought there was little point in their being there since I had killed Windows Defender.
But, being a MF Software Dinosaur, I just had to verify things.

When I get time I will go through the services and start-up items again and report back.

The answer seems to be that one has to accept the "sized down" version of the window.
If any adjustment is made to the "sized down" window then the next time the app is started its window will revert to full screen.