After conferring with @jbphet and @amanda-phet, we agree that the "Undo" buttons do not need to be visible once all three challenges have been answered correctly in a level. @jbphet, the buttons would disappear immediately after completion of a level, just as the Next button appears.
After conferring with @jbphet and @amanda-phet, we agree that the "Undo" buttons do not need to be visible once all three challenges have been answered correctly in a level. @jbphet, the buttons would disappear immediately after completion of a level, just as the Next button appears.
For phetsims/tasks/issues/860.