Messages displayed in the top left corner of the panel almost always overflow into the main display. However, the black tends to disappear because in the early game, there is much undiscovered terrain. It could default to white, but then it could be hard to read on explored terrain. (Not necessarily, it could be possible that white text would be easier to read no matter the terrain.)
The best solution would probably be to have the panel expand horizonally below the messages as long as the player keeps their finger on the panel part where the messages are displayed. This way, the messages would be easy to read when they are needed without getting too much in the way.