WEBP <--------------------------------------THUMBNAIL COMPRESSION-------------------------------------> ZIP
Weirdly, transparency seems to work with lossy compression when copying a png to a new location, but upon refresh, the background turns white. This could be due to the app defaulting to lossless when first moving a file, or it could be a lead. Repeatable steps: start using webp compression, create folder, paste .png in it, alpha should work immediately after pasting, browse to a different folder and back, see that alpha is broken, set compression to zip, rebuild folder cache, see that it works even after folder change, change to webp, rebuild, see that alpha is broken again.
To my eyes, this is obviously a bug, and not a configuration mistake on my end, so I won't ask for a workaround. I doubt this is linked to whatever bug relating to the border size you found back in 2020. Here's the exact .png used in my testing, and not that it should matter, but I was using a thumbnail size of 800x800 the entire time.
Any chance of getting this looked at, officially?
Merci pour le merveilleux logiciel, et tout le travail qu'il nécessite.
[EDIT] xnview.ini: https://paste-bin.xyz/41933
Thanks to your detailed description I can reproduce