OS: Windows 10 - 64-bit
XnViewMP Viewer when zoom mode is set to "Fit window to image" or "Fit image to desktop" causes viewer window to be shifted by 1 pixel up and 1 pixel left on each subsequent image. This does not occur in XnView Classic.
Effect: XnViewMP Viewer when zoom mode is set to "Fit window to image" or "Fit image to desktop" causes viewer window to be shifted by 1 pixel up and 1 pixel left on each subsequent image. The window shifting effect is obvious when you have 2 or more images of the same resolution in the folder and cycle through them. This does not occur in XnView Classic.
To reproduce:
1. from Explorer Window, prepare a folder with 2 or more images of the same resolution, preferably the image width and height is lesser than your desktop resolution to be able to see the -1 pixel window Y movement.
2. double click to open one of the image in XnViewMP viewer (assuming the image file extension is associated with XnViewMP and startup is set to Start with file = Open Viewer). Also make sure the Viewer window is not maximized.
3. In XnViewMP viewer window, Tools menu > Settings or press F12, go to the top section of View settings, set Auto zoom to "Last used", uncheck "Reset 'Auto zoom' settings with next/previous file". click ok to save the settings.
4. Back to XnViewMP viewer window, click on menu View > Zoom > "Fit window to image"
5. Go to the next image with left arrow key or whichever configured key to go to the next image.
6. hold down the go to next image key and observe the Viewer window moves left and up by 1 pixel on each subsequent image loading.
7. click on menu View > Zoom > "Fit image to desktop", go to next or previous image and notice the same happen, Viewer window gets shifted by 1 pixel to the left and up on each subsequent image loading.
8. Additionally you could try this in XnView Classic with Auto Image Size set to "Fit image to desktop, large only" in Tools > Options > View and noticed that the window's position stays which is the correct behaviour.
Actual behaviour (bug): Viewer window's position gets shifted by 1 pixel to the left and up on each subsequent image loading.
Expected behaviour: Viewer window's position should remain on each subsequent image loading.
Thanks to your detailed description I can reproduce