... not screen space

Flip and rotate the canvas around the center of the viewport,
instead of the center of the image.  Scroll the display as
necessary to keep the center of the viewport fixed during
these operations.  This applies to both the corresponding
menu items, and rotation using Shift+Space.

Likewise, flip the canvas across the designated axis in
screen space, instead of image space.  Rotate the display as
necessary, such that the reflection appears to happen in
screen space, regardless of the current rotation angle of the
canvas.