If I maximize the XnViewMP window, then select Menu > View > Hide title bar (Ctrl+F11), then unselect it again, the window will be unmaximized.
Tested with XnViewMP version 0.75 beta2 (2015-06-06) win32 and 0.72 (2014-12-19) win32 on Windows 7 64 bit SP1.
MP: "Hide title bar" on/off unmaximizes window
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