On touchscreen based devices (such as phones like the OpenMoko FreeRunner) that do not have a physical keyboard, the following issues occur with xgalaga, especially when in full-screen mode:
No way to pause the game.
No way to quit an in-progress game.
No way to enter your score after getting a high score. I would suggest that if there is an XCB/Xlib function to request a virtual keyboard, then use that, otherwise just use the user's name (from /etc/passwd) or their username for the high score when no keyboard is available.
As a consequence of the above, there is no way to start a new game or quit after completing a game with a high score.
No way to toggle sound or shields during the game.
In windowed mode, it doesn't resize itself to fit the available area when a virtual keyboard is on screen. IIRC the E17 Illume theme keeps apps maximised.
In windowed mode, it steals the mouse such that you cannot press the keys on the E17 virtual keyboard nor the top panel using the touchscreen (mouse). On my desktop you can't even move the mouse outside the window.
The app renders a mouse cursor even though the device has no mouse (just a touchscreen).