In Windows, when you change the active/inactive border size, the SystemParametersInfo(SPI_GETWORKAREA,....) rect size height (bottom - top) also changes, but in ReactOs it doesn't.
I think the formula Windows uses is this: (Screen height - taskbar height - (border * 2) - 6) for SPI_GETWORKAREA.
I've included a simple program that demonstrates this.
Run the program in Windows (classic theme) and then change the active/inactive border size and hit apply. You'll see that the workarea changes. Now, do the same thing in ReactOs and you'll see that the workarea doesn't change.