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With the GOG enhanced version I have two issues with the mouse.

1: The mouse goes off the edge of the screen and is near unplayable unless the screen is stretched (the same as classic mode).

2: The mouse control feels off, I don't know how the 'improved' version manages to make it harder to play, but it does.

The Classic version works fine for me, I hope these issues get fixed in the future.
I have the same issue, feels like the game forces VSync so there is a slight delay in mouse movements. For me the mouse cursur only goes off the screen when I drag it to the bottom of the screen, all other sides work fine. Would be great if GOG could bring out a hotfix for that.
Same here, mouse and general gameplay is butter smooth in classic, but the enhanced version is sluggish, mouse movement included.
The mouse still goes offscreen for me, but I managed to get it smooth at least by choosing the lowest resolutions that still displays the game perfectly, while also disabling vsync.

For me that is 1280x768. I tried 1280x720, but there was some tearing, but on the former resolution the game looks great and the mouse control is butter smooth. The mouse still disappears offscreen however.