Bug#841642:

[Ayke van Laethem]

This could even be seen as a security issue - the display isn't hidden
at all using the screen lock, the only thing it does is making it
impossible to interact with it. So it is visible what the user was
doing when leaving the computer for some time.

Upstream bug:
https://github.com/mate-desktop/mate-screensaver/issues/103

It appears to be fixed with this patch:
https://github.com/mate-desktop/mate-screensaver/pull/110

It would be nice if this could go into stable stretch.

-- Ayke