I think this has been done some time between stepmania 5 beta 4 and 5.0.5, because I gave a friend of mine some BMSes to try with stepmania 5 and he was using beta 4. The play field appeared on the left hand side as expected.
Is this a bug or was it intentional? Was it a source mod or is it somewhere in the theme? I can seem to be able to move around the play field in any game mode but beat single7, which is especially noticeable in Tony's beatmania IIDX 15 theme, where the playing field appears not where it should be but in the middle of the screen.