I am able to drag the Legend icon from an open message in a Shortwave browser pane to the the outline in an adjacent pane, BUT it only works if I drag it in between two items (with the horizontal line insert indicator showing) or onto an EXPANDED outline item (with the rectangle indicator showing). If I drag it onto a COLLAPSED item, a blank child item is added where I would expect the Shortwave message to be inserted (ie, the last child item).
I’ve never been able to drag straight from the inbox list, as you describe.