Folder thumbnail should be updated automatically when files changed

[fireattack]

This has been bothered me for years, I think it's more like a bug than a feature request so I put it here, but feel free to move.

When you enable thumbnail for folder (in my case, I use "Show 4 thumbnails" option too),
and you move files in and out the folder, it doesn't automatically refresh.

This is the most obvious when you have a folder that used to have several files and now is empty, and vice versa (empty and now have files).
This is very confusing visually, and I had to constantly manually rebuild the thumbnail to have it updated, so I can know which folder is actually empty at a glance. This kind of defeats the point of having thumbnails for folders.

I think this should automatically refresh if the file changes can be detected (which should be trivial when you move files inside XnView MP itself.)
It does not need to rebuild the thumbnail itself per se if it's cached, just need to update to show the ones that are still within the folder.