Cannot extend compound clip duration

Already many years ago there have been discussions about FCPX's unability to extend the duration of a compound clip. It was then suggested this would become possible in future versions of FCPX, but it's still not there. For some weird reason a compound clip, created from a selection in the time line, has a fixed duration. Adding content later to this CC will create a dark area past the existing end point which will not be shown in the time line. The end point is marked by a little triangle, which invites to be dragged to the right to extend the clips duration, but you cannot do anything with it. In fact, why is it even there ?? Adding content should automatically extend the clip.

Proposed solutions the extend the length of the CC in the time line by dragging the yellow handles simply don't work.

So, the question remains: how do I add content to a CC and get rid of the dark area ?