  Time-reversal ($\mathcal{T}$-) symmetry is fundamental to many physical
processes. Typically, $\mathcal{T}$-breaking for microscopic processes requires
the presence of magnetic field. However, for 2D massless Dirac billiards,
$\mathcal{T}$-symmetry is broken automatically by the mass confinement, leading
to chiral quantum scars. In this paper, we investigate the mechanism of
$\mathcal{T}$-breaking by analyzing the local current of the scarring
eigenstates and their magnetic response to an Aharonov--Bohm flux. Our results
unveil the complete understanding of the subtle $\mathcal{T}$-breaking
phenomena from both the semiclassical formula of chiral scars and the
microscopic current and spin reflection at the boundaries, leading to a
controlling scheme to change the chirality of the relativistic quantum scars.
Our findings not only have significant implications on the transport behavior
and spin textures of the relativistic pseudoparticles, but also add basic
knowledge to relativistic quantum chaos.
