You could do that without overlapping windows. Eg. Pick up the object, drag it to a screen edge so the multitasking view is being peeked and then subsequently shown, hover over the app and drop the object on the cover or if you hover long enough it could spring open the app so you can drop the object directly into the now full screen app.

That's how desktop Mac OSX handles drag and drop today onto multiple virtual desktops, screens and minimised apps.

I'm not saying Jolla should do that; just saying a windowed environment isn't a prerequisite.