There's a small bug with the Photosynth 2 viewer that's been there for months and is still there after the recent update.
Steps to reproduce:
:: 1 Begin with a windowed viewer (a smaller window will make this more obvious - say half the height + width of your monitor).
:: 2 Use your web browser's own fullscreen mode (F11 on most Windows browsers).
:: 3 Use Photosynth's own fullscreen icon or the [F] key to enter the Photosynth 2 viewer's own fullscreen mode.
:: 4 Use Photosynth's own fullscreen icon or the [F] key to exit the Photosynth 2 viewer's own fullscreen mode.
Result: the viewer redraws at the original windowed resolution but floating in the empty space afforded by the web browser's own fullscreen mode, docked to the upper left corner of the window/tab.