Debian Stretch Xfce: effective screeen bigger than the monitor screen

[Jerome BENOIT <sphericaltriangle@rezozer.net>]

Hello Forum,

I have recently migrated from Jessie to Stretch. I have a couple of minor issues.
One of them concern Xfce: some time my fingers seem to do a combination of keys
that magnifies the screen: I got a floating desktop environment that is lager
than my actual screen. Does any know the magic combination that leads to this state ?
And, more importantly, how can we go back to the normal state ?

Thanks in advance,
Jerome