The current movewindow dragmove handler overwrites the window flags in the call to WindowPosition, so any transparent miniwindow loses its transparency as soon as it gets dragged.
Proposed fix: https://github.com/nickgammon/mushclient/pull/68/commits/baec90bce530809307ae612d89741c57bb237492
Proposed fix: https://github.com/nickgammon/mushclient/pull/68/commits/baec90bce530809307ae612d89741c57bb237492