HP was true to their word by calling it an emulation window, where it will run a 320 x 480 resolution window (just like a Pre). Being the first released draft of the Mojo emulator, it must be forgiven when there are some noticeable quirks, such as it being sideways relative to the menu bar, sporting a simulated phone around it. There is also a gesture area located below which will respond to forward and back swipes, and we do speculate that it will also do so for meta-taps.
Documentation thrown into the Beta 5 emulator mentions that Mojo apps “compatibility mode” will run by default at 320 x 480 resolution. Are you excited about webOS 3.0 already?