  We report the detection of FRB20191107B with the UTMOST radio telescope at a
dispersion measure (DM) of 714.9 ${\rm pc~cm^{-3}}$. The burst consists of
three components, the brightest of which has an intrinsic width of only 11.3
$\mu$s and a scattering tail with an exponentially decaying time-scale of 21.4
$\mu$s measured at 835 MHz. We model the sensitivity of UTMOST and other major
FRB surveys to such narrow events. We find that $>60\%$ of FRBs like
FRB20191107B are being missed, and that a significant population of very narrow
FRBs probably exists and remains underrepresented in these surveys. The high DM
and small scattering timescale of FRB20191107B allows us to place an upper
limit on the strength of turbulence in the Intergalactic Medium (IGM),
quantified as scattering measure (SM), of ${\rm SM_{IGM} < 8.4 \times 10^{-7}
~kpc~m^{-20/3}}$. Almost all UTMOST FRBs have full phase information due to
real-time voltage capture which provides us with the largest sample of
coherently dedispersed single burst FRBs. Our 10.24 $\mu$s time resolution data
yields accurately measured FRB scattering timescales. We combine the UTMOST
FRBs with 10 FRBs from the literature and find no obvious evidence for a
DM-scattering relation, suggesting that IGM is not the dominant source of
scattering in FRBs. We support the results of previous studies and identify the
local environment of the source in the host galaxy as the most likely region
which dominates the observed scattering of our FRBs.
