This is Sushi, a quick previewer for Files (nautilus), the GNOME desktop file manager.

Sushi is a DBus-activated service. For Sushi to popup, applications will have to call the ShowFile method on the org.gnome.NautilusPreviewer2 interface.

If you want to debug Sushi without a calling application, you can use the sushi <filename> command.

Before filing an issue here, confirm that it has not been fixed on the latest nightly build. The easiest way to do this is by installing the nightly flatpak.

flatpak remote-add --if-not-exists gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo
flatpak install gnome-nightly org.gnome.NautilusPreviewer.Devel

After installing the nightly flatpak, you can launch Sushi via the below command, or by initiating a preview from the NautilusDevel nightly flatpak.

flatpak run org.gnome.NautilusPreviewer.Devel <filename>

See the plugins subfolder.