/usr/share/applications/screensavers/halftone.desktop is in xscreensaver-data-extra 5.36-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | [Desktop Entry]
Name=Halftone
Exec=/usr/lib/xscreensaver/halftone -root
TryExec=/usr/lib/xscreensaver/halftone
Comment=Draws the gravity force in each point on the screen seen through a halftone dot pattern. The gravity force is calculated from a set of moving mass points. View it from a distance for best effect. http://en.wikipedia.org/wiki/Halftone Written by Peter Jaric.
StartupNotify=false
Terminal=false
Type=Application
Categories=Screensaver;
OnlyShowIn=MATE;
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