Can files or folders be dragged to the title bar of a lister (as they can be to an Explorer window's title bar)? At the moment I get the "prohibited" icon when trying to do that, and I can't find an option to enable it.
I've always found that behaviour useful, since the title bar is often visible when the rest of the window is not, and dragging to the title bar guarantees that the files will go into the current folder, rather than mistakenly ending up in a subfolder.
You can't drag to the titlebar, but if you're worried about accidentally dragging into sub-folders, you can turn off Preferences / File Displays / Mouse / Allow drag and drop into sub-folders (without a qualifier key held down).
Yeah, I did see that option.
Could you maybe suggest dragging to the title bar as an optional feature for a future release? Am I the only one that does this? 
I miss this behaviour found in File Explorer.
It would be nice if dropping to the title bar would drop the dragged-in item to the active file display.
Being able to do so means not having to have so much of the lister visible on screen (i.e. the file display itself does not to be visible to drop an item in the foreground to the window in the background, which aids multitasking).
Yep
If you use Dual Lister mode, I'd be curious to know whether dropping a file (or files) on the titlebar should put the file(s) in the Source lister or the Destination lister.
I could see arguments for either side being valid. The Destination lister has obvious reasoning. But I think many people consider the Source lister to be the "active" portion of their DOpus window.
This is something to think about when implementing such a feature and it might be one reason why it hasn't been implemented.
Good points. I could see either choice being confusing. The titlebar usually shows the source path, but even that can be edited.
It would also probably need to be turned off when customizing toolbars, else it'd be too easy to start creating stray copies of items you were trying to move around on the menu bar (top toolbar) if you weren't careful.
(I'm also not sure if would require a lot of refactoring, since currently each tree and file display handles its own drop target. If the whole top-level window became a drop target, that logic may need to be moved into a common place which negotiates with the child windows. Or maybe not. I'm not actually sure Windows handles multiple overlapping drop targets, without doing some research. I know it causes problems with some UI frameworks.)
If you're copying things between two top-level Opus windows, when the second usually isn't visible, you can use Copy to=ask to ask for a destination, which includes a pop-up list of currently open folders in other Opus windows.
You can also drop on to the folder tabs at the bottom of the window, if you want a drop target that's nearer a window edge.
This is a decent suggestion, however even after moving my Folder Tabs placement back to the bottom (the default I believe), it doesn't help much in Dual Horizontal mode (my preference) as the top Folder Tabs row is still in the middle of the window. Your suggestion is best in Dual Vertical mode. Being able to drop to the title bar would be ideal, but it's understandable if there are some issues implementing it.
Being able to drop on the titlebar might not help much in dual horizontal mode either, since you'd only be able to drop on one side or the other. (Unless we started custom-drawing the titlebar, which we want to avoid if possible.)
I believe you may have meant Dual Vertical.
I'm quite new to using DOpus but to my mind, it's as simple as providing a preference option such as "Dropping to title bar drops to: source|destination". That would mean the behaviour is explicitly defined.
Perhaps the title bar string could temporarily change to read "Drop to: /path/of/src_or_dest" (whichever is configured), that way any custom title bar string couldn't cause confusion.
I'm not sure how well it would work, the more I think about it.
You would have to make sure the side you wanted to drop in was (or wasn't) active in advance for that to work. Do you always make sure you click the left (or right) file display before moving from one window to another?
More generally, is the issue with dragging from other software to Opus, or is it Opus-to-Opus? There are lots of ways to copy things in Opus without needing drag & drop or a second window at all.
I haven't found myself needing more than one Lister, so I typically only drag from the Desktop, File Explorer, or from within an archive (i.e. 7-zip) to Opus.
Since I'm usually doing a specific task when moving files around, I typically know which file display is currently active, however mistakes could still happen from time-to-time. The title bar string dynamically changing would do a lot to minimise those mistakes I believe.
All of those can be done from Opus? (Unless you're using 7-Zip on something esoteric, like installers, a lot.)
What I've found in practice is that it's not all that easy to copy to the wrong folder because, even when you have so many subfolders that the entire lister is only showing subfolders, there are large areas where the drag 'n' drop target is the parent folder. However, there an interesting reason for this compared to the Explorer: the column width of subfolders (in list mode) is determined by the width of the longest filename, whereas column widths in Explorer seem to be variable. This makes the empty space between folder names in Directory Opus larger.