Did you try changing the game resolution in the configuration program, or switching to software mode, rather than Direct3D?
Does the selected resolution match the aspect ratio of your monitor?
In the Divine Divinity install folder, try right clicking on the div.exe program file, select Properties, switch to the Compatibility tab and try different compatibility modes. In Win 7, disable desktop composition and check 'Disable display scaling on high DPI settings'; for Win 10, check 'Override high DPI scaling behavior' and set it to Application.
Try resetting your graphics driver's 3D settings to default (right click an open area of the desktop to start the nvidia Control Panel / AMD Radeon Settings), and check if there are any options there for stretching or scaling fullscreen applications.
Try the Windows 8 Performance "Patch"
(works in Win 7 and 10, as well).
You can try running the game in windowed mode. This can be done by hex editing the div.exe executable in the install folder (see the topic [Info] Debug Mode - Better FPS + Windowed Mode
), or download this edited version
and extract div-window.exe (Steam v1.0062) into your Divine Divinity folder and try running it from there.