  The portal connecting the invisible and visible sectors is one of the most
natural explanations of the dark world. However, the early-time dark matter
production via the portal faces extremely stringent late-time constraints. To
solve such tension, we construct the scalar-controlled kinetic mixing varying
with the ultralight CP-even scalar's cosmological evolution. To realize this
and eliminate the constant mixing, we couple the ultralight scalar within
$10^{-33}\text{eV} \lesssim m_0 \ll \text{eV}$ with the heavy doubly charged
messengers and impose the $\mathbb{Z}_2$ symmetry under the dark charge
conjugation. Via the varying mixing, the $\text{keV}-\text{MeV}$ dark photon
dark matter is produced through the early-time freeze-in when the scalar is
misaligned from the origin and free from the late-time exclusions when the
scalar does the damped oscillation and dynamically sets the kinetic mixing. We
also find that the scalar-photon coupling emerges from the underlying physics,
which changes the cosmological history and provides the experimental targets
based on the fine-structure constant variation and the equivalence principle
violation. To ensure the scalar naturalness, we discretely re-establish the
broken shift symmetry by embedding the minimal model into the
$\mathbb{Z}_N$-protected model. When $N \sim 10$, the scalar's mass quantum
correction can be suppressed much below $10^{-33}\text{eV}$.
