Slideshows now have a white border on the left, right, and bottom in PM6 and PM6+. It's not bothersome if I'm just using my computer screen, but I do most slideshows on a big TV and it really noticable. This occurs in both full-screen and windowed mode.
For the screenshot I put it in windowed mode on a black background so it can been seen clearly.
build 3783
macOS 10.14.6