  We prove in the setting of $Q$--Ahlfors regular PI--spaces the following
result: if a domain has uniformly large boundary when measured with respect to
the $s$--dimensional Hausdorff content, then its visible boundary has large
$t$--dimensional Hausdorff content for every $0<t<s\leq Q-1$. The visible
boundary is the set of points that can be reached by a John curve from a fixed
point $z_{0}\in \Omega$. This generalizes recent results by
Koskela-Nandi-Nicolau (from $\mathbb{R}^2$) and Azzam ($\mathbb{R}^n$). In
particular, our approach shows that the phenomenon is independent of the linear
structure of the space.
