Background is not black in full screen mode, but is in windowed mode

[Domarius]

If I double click the image to go full screen, I get black above and below, but these white bars on the sides

But if I double click and return to window mode, I see this - completely black background all round

My settings look like this

I'm on Windows 10, and my XnView MP version is: 1.5.5 64-bit (Windows) - Libformat 7.155 (a button to copy that to the clipboard would be good!)

[user0]

"black above and below" are in the image itself

to change bg color in fullscreen goto
Settings > View > Fullscreen

[Domarius]

"black above and below" are in the image itself

to change bg color in fullscreen goto
Settings > View > Fullscreen

Ah... it seems you were right on both counts. Thank you for clearing that up!