  We study nonequilibrium properties of an electronic Mach-Zehnder
interferometer built from integer quantum Hall edge states at filling fraction
$\nu{=}1$. For a model in which electrons interact only when they are inside
the interferometer, we calculate exactly the visibility and phase of
Aharonov-Bohm fringes at finite source-drain bias. When interactions are
strong, we show that a lobe structure develops in visibility as a function of
bias, while the phase of fringes is independent of bias, except near zeros of
visibility. Both features match the results of recent experiments [Neder
\textit{et al.} Phys. Rev. Lett. \textbf{96}, 016804 (2006)].
