Timeline: Percussion instruments are always grayed out
OS: OS X El Capitan (10.11), Arch.: x86_64, MuseScore version (64-bit): 188.8.131.5282, revision: 1f7a1f8
Percussion instruments are excluded from the blue lasso border indicating which bars / instruments are displayed in the main pane. I tested with two, tambourine and claves.
It makes no difference where in the order the percussion instrument is, between other instruments or at the end.