You can transfer parts into the part comparison via Drag & Drop:
As of V10 SP2 there are two different modes:
If you release the mouse key within a second over the tab Part comparison the part is overtaken but there is no change-over to the part comparison so that you can select further parts.
If you stay over the tab Part comparison longer than a second, the application switches into the part comparison. If you then release the mouse key the part is overtaken into the part comparison.

