  The vast majority of close binaries containing a compact object form through
common-envelope (CE) evolution. Despite this importance, we struggle to even
understand the energy budget of CE evolution. For decades, observed long-period
post-CE binaries have been interpreted as evidence for additional energies to
contribute during CE evolution. We have recently shown that this argument is
based on simplified assumptions for all long-period post-CE binaries containing
massive white dwarfs. The only remaining post-CE binary star that has been
claimed to require contributions from additional energy sources to understand
its formation is KOI 3278. Here we address in detail the potential evolutionary
history of KOI 3278. In particular, we investigated whether extra energy
sources, such as recombination energy, are indeed required to explain its
existence. We used the 1D stellar evolution code MESA to carry out binary
evolution simulations and searched for potential formation pathways for KOI
3278 that are able to explain its observed properties. We found that KOI 3278
can be explained if the white dwarf progenitor filled its Roche lobe during a
helium shell flash. In this case, the orbital period of KOI 3278 can be
reproduced if the CE binding energy is calculated taking into account
gravitational energy and thermodynamic internal energy. While the CE evolution
that led to the formation of KOI 3278 must have been efficient, that is, most
of the available orbital energy must have been used to unbind the CE,
recombination energy is not required. We conclude that currently not a single
observed post-CE binary requires to assume energy sources other than
gravitational and thermodynamic energy to contribute to CE evolution. KOI 3278,
however, remains an intriguing post-CE binary as, unlike its siblings,
understanding its existence requires highly efficient CE ejection.
