With the default keymapping, holding
SHIFT to smooth and then additionally holding down the
ALT key (on a PC keyboard in the browser version) switches back to the active brush.
I think it would be more intuitive that with
ALT both down, smooth with the alt mode (relax) could turn on.
CTRL (mask) and
ALT (unmask) together switches back to the brush, instead of unmasking.
(This could be a browser version thing again, would be nice to have access to the current version as a browser version to compare)