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I don't know why GOG can't get the 'improved version' on Steam, but if they can't they should consider not selling it.

"New Enhanced Edition for Steam!

High-quality, remastered OST
Original bonus Comic Book included
Improved mouse support - You can rebind left and right click function to any mouse button; scroll wheel can be used to navigate in-game menus and comic books
Improved keyboard support - You can rebind virtual left and right click keys; page up and page down act the same as the mouse scrollwheel. Can use number keys to make choices during dialogue trees. Can adjust sensitivity of virtual pointer.
Support for modern displays with ability to toggle between fullscreen and windowed mode
SDL 2.0 Game controller support added, with rebinding and sensitivity controls supported.
Several game crashing bugs from the original code were fixed.
Dead-end situations in game play were removed.
Optional linear texture filtering.
Three-band equalized, filtered sound mixer."
AFAIK the only version ever made available here is the enhanced one.