Select the top case half.
Move the mouse into the thumbnail bottom right and down the smart selection ladder one level from the active level, then double-click.
Dynamic highlighting shows you the active level and parts not in this level are transparent.
Start to drag the bottom face of the top half of the plastic case towards the bottom half, and then release the left mouse button.
The top half of the case is attached to the pointer.
Move the mouse pointer over the top face of the bottom half then click.
The two parts snap together. Notice, however, that the fit is not perfect.
When finished, click the background.