Ensure you are using the correct resolution for the game. Check the video card's GPU scaling and overscan settings. If you are using a TV as the display you may also need to adjust the TV's own settings.
Some games have a window border visible in fullscreen mode on Windows 8 and later. Most games sold on GOG.com are already patched to fix this issue. Refer to the individual game pages for pre-made fixes for many games. To fix this manually install the Microsoft Application Compatibility Toolkit and apply the ForceSimpleWindow compatibility fix.
This issue occurs on Windows 8 and later when playing certain older games. Most games sold on GOG.com are already patched for this when needed. See the individual game pages for pre-made fixes for many affected games. This can be resolved manually by using the Compatibility Administrator to make a compatibility fix with the DXPrimaryEmulation -DisableMaxWindowedMode parameter.
This behaviour affects DirectX 8 and earlier. Some games can be fixed by using a wrapper, such as d3d8to9 [Note 1] or dgVoodoo 2 [Note 2].