Audacity 1.3.11 and 1.3.12 on Ubuntu Linux (probably other versions and platforms also but I've not tested).

The Time Shift tool can be used to drag a track or "audio clip" to another track provided that:

a) the track is of the same type (mono/stereo audio track)
b) there is sufficient room for the clip
c) the clip being dragged is NOT selected (and no part of it is selected).

Why the last condition?
Is this an intended behaviour, an unintentional limitation or a bug?