I've been a happy XnView user for almost 20 years.
Lately, I've been pondering the big switch to Xnview MP, so I've installed them both side by side and spent the weekend trying to configure everything to my liking. This is gonna be a series of my findings, suggestions and bug reports.
MP toolbars take up more vertical space than Classic.
I've taken screenshots of Classic vs MP and stitched them together.
While the album cover fits perfectly in Classic (Zoom = 100%), it needs to be scaled down in MP (Zoom = 99%), because the MP toolbar and MP infobar are taking up more vertical space.

-> to see more details open screenshot in new tab
Classic toolbar: 81 pixels vs MP toolbar: 87 pixels
Classic infobar: 25 pixels vs MP toolbar: 26 pixels
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Classic total: 106 pixels vs MP total: 113 pixels
In total, MP is taking up 7 pixels more vertical space than Classic.
-> Could you slim down the MP bars?