  We report preliminary results from an analysis of the pion scalar form factor
computed on a set of the $\mathrm{tr}[M]=\mathrm{const}$ CLS gauge ensembles
with $N_f=2+1$ Wilson Clover-improved sea quarks. The calculations are carried
out for light quarks masses corresponding to $M_\pi\approx 0.130\mathrm{MeV}
\ldots 350\mathrm{MeV}$, four values of the lattice spacing
$a\approx0.049\mathrm{fm}\ldots0.086\mathrm{fm}$ and a large range of physical
volumes. A fine-grained momentum resolution is achieved by allowing for
non-vanishing sink momenta and by including two particularly large and fine
boxes close to physical quark masses (i.e. $T\times L^3 = 192\times 96^3$,
$M_\pi \le 172\mathrm{MeV}$, $a \le 0.064\mathrm{fm}$). The pertinent
quark-disconnected contributions have been computed to high precision using a
scheme combining 1.) the one-end trick on stochastic volume sources for the
computation of differences between two quark flavors with 2.) the hopping
parameter expansion and hierarchical probing to evaluate the loops for the
heaviest, single quark flavor.
