Zoom in and fill the screen
Zoom into a photo up to twenty times, show it edge to edge or as a floating card, and go full screen with F11.
Floating card or edge to edge
By default the photo floats as a card at about 90% of the window, with rounded corners and a shadow, so the blurred grid shows around it. Press F to switch to edge-to-edge, where the photo fills the window, and F again to go back. The choice is remembered, so the next viewer opens the same way. It can also be set under Appearance as Viewer: edge-to-edge fit (F).
Zooming in
Use the mouse wheel over the photo to zoom in, up to twenty times, and drag to move around the enlarged photo. Double-click while zoomed to go back to the whole photo. While the photo is not zoomed, a single click closes the viewer.
Full screen
F11 makes the whole window fill the screen, which combines well with edge-to-edge fit. Press F11 again, or use the ✕ in the corner, to leave.
Rotation
R and Shift+R turn the photo, and the card turns and changes shape with it. Videos and RAW files cannot be rotated.