  Gamma-ray bursts are produced as a result of cataclysmic events such as the
collapse of a massive star or the merger of two neutron stars. We monitored the
position of the close-by gamma-ray burst GRB~190829A, which originated from a
massive star collapse, through very long baseline interferometry (VLBI)
observations with the EVN and the VLBA, involving a total of 30 telescopes
across 4 continents. We carried out a total of 9 observations between 9 and 117
days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of
few milliarcseconds (mas). We obtained limits on the source size and expansion
rate. The limits are in agreement with the size evolution entailed by a
detailed modelling of the multi-wavelength light curves with a forward plus
reverse shock model, which agrees with the observations across almost 18 orders
of magnitude in frequency (including the High Energy Stereoscopic System data
at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to
the broad, high-cadence coverage of the afterglow, afterglow degeneracies are
broken to a large extent, allowing us to capture some unique physical insights:
we find a low prompt emission efficiency $\lesssim 10^{-3}$; we constrain the
fraction of electrons that are accelerated to relativistic speeds in the
forward shock to be $\chi_e<13\%$ at the 90\% credible level; we find that the
magnetic field energy density in the reverse shock downstream must decay
rapidly after the shock crossing. While our model assumes an on-axis jet, our
VLBI astrometric measurements alone are not sufficiently tight as to exclude
any off-axis viewing angle. On the other hand, we can firmly exclude the line
of sight to have been more than $2\,\mathrm{deg}$ away from the border of the
region that produced the prompt gamma-ray emission based on compactness
arguments.
