Rocks'n'Diamonds

I found the following:

currently, original Supaplex levels that are played with the Supaplex engine have a thick gray border consisting of Supaplex blocks. In the original game, this border is a thin, uninterrupted grey line. Can't this be imitated so that it looks more like the original?

It's already there:

Setup->Graphics->Supaplex Border Elements: off

I'm a bit confused now, because "off" is the default setting and should only be "on" if you changed it to that value in the setup menu. :-o

(Explanation for non-Supaplexers: The border elements (usually gray blocks) are part of the level playfield, but are usually not displayed by the (original) Supaplex game, but a solid, half-tile-sized border is displayed instead. There exist levels using dirty tricks by replacing these (usually invisible) border elements by collectible or walkable game elements, which could be used to hide infotrons to be collected or even to create wrap-around levels. To make these border elements visible, set the above option to "on".)

Ah great. Well, I have never changed it, but it was set to "on" in my version, that's why. Maybe you should check it one time over if it is really set to 'off' by default, but otherwise it was just something in my RnD version only.

To check that "off" is the default, just rename the file "setup.conf" (or remove the line containing "sp_show_border_elements" from it) and restart the game. No idea why it was set to "on" on your side... Strange.