2Troken
Yes, that is
exactly the "combined" tree idea I was promoting. I use Media Monkey also and was planning to post essentially that screenshot..
The multiple collapsible panels idea is different...
will post a screenshot from Adobe Lightroom Beta 4 later today, but you can try that software out by signing up for free.
2Pierre
Perhaps i don't understand, but it's always one tree with 3/4 root entries???
Yes, it behaves that way, though a collapsible panel doesn't actually have to have a (sub) tree in it. This kind of approach is in use for many years in 3-D modeling software (3DS Studio Max was one of the first).
Another good paradigm for this sort of thing (panels, sidebars) is PaintShopPro (version 8 and above). It has a very dynamic approach to sidebars with drag-and drop of panels into/out of the sidebars. Sidebar panels can collapse to vertical tabs, or float over the image, or dock inside the sidebar in any position and any size you want.
But...
That's more flexibility than I think is strictly necessary for XnView, hence my original suggestion of a "combined" tree, instead.
Solving this trees vs tabs vs panels layouts thing for XnView
once-and-for-all would make enhancing XnView much easier in the future, though I realize that here is a LOT of upfront design and programming work involved.