  We provide an overview of the current state of lattice calculations
concerning isospin-breaking corrections in weak decays, focusing in particular
on leptonic decays of light pseudoscalar mesons. We examine the two currently
existing calculations, placing a particular emphasis on the role of
electromagnetic finite-volume corrections. Given the large systematic
uncertainty associated with such corrections, we propose a novel method to
improve the finite-volume scaling of leptonic decay rates and other hadronic
observables, like hadron masses and the hadronic vacuum polarization
contribution to the muon $g{-}2$. We introduce a new prescription for the QED
action, referred to as QED$_\mathrm{r}$, which effectively removes
finite-volume corrections at $\mathcal{O}(1/L^3)$ and consequently shifts the
systematic uncertainty to a higher order. The specifics of this action, the
practical implementation of the infrared improvement, and the ongoing numerical
tests are also discussed.
