  We demonstrate how the least luminous galaxies in the Universe, ultra-faint
dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic
reionization. We select a low-mass ($\sim \text{1.5} \times 10^{9} \,
\text{M}_{\odot}$) dark matter halo from a cosmological volume, and perform
zoom hydrodynamical simulations with multiple alternative histories using
"genetically modified" initial conditions. Earlier forming ultra-faints have
higher stellar mass today, due to a longer period of star formation before
their quenching by reionization. Our histories all converge to the same final
dynamical mass, demonstrating the existence of extended scatter ($\geq$ 1 dex)
in stellar masses at fixed halo mass due to the diversity of possible
histories. One of our variants builds less than 2 % of its final dynamical mass
before reionization, rapidly quenching in-situ star formation. The bulk of its
final stellar mass is later grown by dry mergers, depositing stars in the
galaxy's outskirts and hence expanding its effective radius. This mechanism
constitutes a new formation scenario for highly diffuse ($\text{r}_{1 /2} \sim
820 \, \text{pc}$, $\sim 32 \, \text{mag arcsec}^2$), metal-poor ($\big[
\mathrm{Fe}\, / \mathrm{H} \big]= -2.9$), ultra-faint ($\mathcal{M}_V= -5.7$)
dwarf galaxies within the reach of next-generation low surface brightness
surveys.
