  A kinematically complete quasi-free $(p,pn)$ experiment in inverse kinematics
was performed to study the structure of the Borromean nucleus $^{17}$B, which
had long been considered to have neutron halo. By analyzing the momentum
distributions and exclusive cross sections, we obtained the spectroscopic
factors for $1s_{1/2}$ and $0d_{5/2}$ orbitals, and a surprisingly small
percentage of 9(2)$\%$ was determined for $1s_{1/2}$. Our finding of such a
small $1s_{1/2}$ component and the halo features reported in prior experiments
can be explained by the deformed relativistic Hartree-Bogoliubov theory in
continuum, revealing a definite but not dominant neutron halo in $^{17}$B. The
present work gives the smallest $s$- or $p$-orbital component among known
nuclei exhibiting halo features, and implies that the dominant occupation of
$s$ or $p$ orbitals is not a prerequisite for the occurrence of neutron halo.
