Drag and Drop a PDF onto the "RS" logo or the "Explorer" file view (I think anywhere other than the sanctioned "upload" box in the references panel).

The window turns into a PDF viewer and there's no way to get back to the RefStudio app other than closing it and re-opening.

We should either preventDefault on all drag-and-drop interactions in the Tauri window or allow PDFs to be dropped anywhere to import them.