/usr/share/applications/gtk3-widget-factory.desktop is in gtk-3-examples 3.22.30-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Name=Widget Factory
Comment=A showcase for GTK+ widgets, designed for testing themes.
Exec=gtk3-widget-factory
Icon=gtk3-widget-factory
Terminal=false
Type=Application
StartupNotify=true
Categories=Development;GTK;
NoDisplay=true
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