I am working with Scrivener on a very large serial project and use massive the binder and collections to manage my publication formats and produce appropriate outputs.
With over 100 collections, creating new collections becomes increasingly unwieldy as the collection display always covers the binder. If I want to add new documents, I first have to go back to the Binder view to drag and drop the files into the new collection, which is often outside the display window in the upper panel.
It would be a great relief for the entire workflow if the binder and collections were decoupled, as two individual display panels, ideally next to each other, so that one can comfortably select documents in the binder and simply drag them over to the collection. I have created an example with Photoshop to illustrate what I mean.
This is very easy with Keyboard Maestro. And if you work like you do, you should really use this app in combination with Scrivener. I couldn’t do without it anymore.
You can create a fully customizable floating palette (always on top and visible), position it anywhere you want and execute any command from Scrivener with one click.
In your case: you want to have your collections always visible in a separate window. Here is an example, which I have colored like your Photoshop image.
Behind each collection is a simple Keyboard Maestro macro that moves a document selected in the binder to the corresponding collection. You just have to click on the name of the collection in the palette.
The opposite way works too—you could load the collection in the editor (via the arrow icon on the left of the collection header), lock it in place (Ctrl-click the editor header to select the setting), and drag items into it. And with the ability to link the editors, in either case you could set up navigation to allow clicks in the outliner to load the document in the split editor, so you could see a binder view, collection view, and your working document all together.
This was implied by @fto’s comment, but just in case you weren’t aware, you can add items to a collection via the menu, not just by drag and drop. In the main menu, this is Documents ▸ Add to Collection ▸ with the list of collections in the submenu, but it’s also in the context menu if you Ctrl-click an item (or multiple selection) in the binder, outliner, or corkboard.
You also can rearrange collections by dragging and dropping the tabs into the order you want, so if you tend to be working with a particular collection for a time, you could drag it to be right above the Binder tab, keeping it more easily accessible.