This bug was introduced in commit 10680 and caused in MixerTrackCluster::UpdatePan(), by using USE_MIDI (which is on in releases) instead of EXPERIMENTAL_MIDI_OUT (which is off). It checked whether a NoteTrack pointer was NULL, and if not, hid the Pan slider (not used in NoteTrack clusters). But an invalid pointer is non-null, so was causing the Pan slider to always be hidden.
final but it's a big improvement. Reversed a couple changes from
original indication scheme that are no longer necessary. Unfortunately
it's really hard to remove colors from AllThemeResources.h.