I remember that Jazz2 originally had a max resolution of 640x480. However, Jazz2+ upped the max screen resolution to 800x600, which is great. Now on my 1080p displays I can do 2x integer scaling with the DirectDraw wrapper dgVoodoo2, and thus choose the resolution of 800x540 to fill most of the real estate of my 1080p displays.

With the resolution height of 540, I can with a 2x integer scale fill my entire screens height. With the resolution width of 800 however, I can fill way more of my screens width than I could with vanilla Jazz2 using 2x integer scaling, but there's still some black bars left on both sides. If the resolution limit for the screens width were upped to 960 however, I could use the resolution of 960x540, and with 2x integer scale that would be 1920x1080, pixel perfect.

I have no idea of how Jazz2+ managed to do add all its feature without the actual source code of Jazz2, but I just wonder if there's hard-coded engine limitations of Jazz2 that made the decision of not adding higher resolutions than 800x600 in Jazz2+?

There are one or two engine limitations that happened to be compatible with 800x600 that would not be compatible with higher resolutions--all fixable--but mostly it's a principled decision, e.g.

• The bigger the game window, the slower it is to draw, so the lower your FPS.
• The bigger the game window, the more of the level you can see around you at any given time, which has massive effects on gameplay, especially multiplayer gameplay. If different people have different resolutions, whoever has the bigger window is going to be able to see farther ahead of them, gaining a huge competitive advantage.
• Jazz's sprites get comparatively tinier and tinier as the window gets bigger. Besides this being less attractive, psychologically it feels like Jazz runs slower when he takes up less of the window, you move across a smaller percentage of the screen in a constant period of time.
• All existing levels are designed for existing maximum resolutions. There are many levels designed for 640x480 that look bad when played at 800x600, usually because one or more of their parallax background layers cut off at the bottom. Increasing the maximum resolution again would give us another round of the same problem.
• In local single player, objects are activated and deactivated based on how close they are to the local player. Increasing the maximum resolution much more would cause them to visibly activate and deactivate at the edges of the window. We can't change that distance because many levels rely on it for puzzles and such.

I have no idea of how Jazz2+ managed to do add all its feature without the actual source code of Jazz2

Not related to the main thrust of this thread, but we do have some of the source code, though unfortunately none of the sections related to window size.