I'm usinng Xfce4. mupdf opens unmaximized, but with its window decoration
indicating it in maximized state (two overlapping windows). Hitting F10 leaves
the window in unmaximized state, but changes the decoration to correctly show
this (one window). Hitting F10 again does the correct thing (maximizes the
window, and changes the decoration correctly to two overlapping windows).
Subsequently, everything behaves correctly, with F10 toggling the window (and
its decoration) between maximized and unmaximized states.