Drag a file onto the shelf from one window. Drag it off into another, whenever you get there. The two windows never have to be open at the same time, let alone side by side.
DropDock holds references, not copies. Nothing is moved, renamed or duplicated when you drop it on the shelf, and removing something from the shelf never touches the file.
A narrow always-on-top strip that holds the handful of files you are working with right now. It lives in the notification area, so it is out of the way until you want it.
DropDock is worth having if you reuse the same few files across a working session — attaching one document to three emails, dropping the same asset into two applications, moving things out of Downloads as they arrive.
If your files go one way once and are never touched again, a shelf saves you nothing. It is also not a file manager: no folders, no tags, no renaming, no previews, and none of those are planned. It holds a short list of things you are using right now, and that is the whole product.
Two limits worth knowing before you install rather than after:
DropDock is not publicly listed yet. It is going into closed beta on the Microsoft Store. If you would like an invitation, write to dropdock.support@gmail.com.