broken sword 5 the serpents curse

there should be a "vt7.ini" file in the folder where the saves sit (Library/Application Support/Broken Sword 5 on a Mac). If you edit that there should be a line that says "fullscreen=1". Try changing that to "fullscreen=0" or "fullscreen=2" and then running the game again...

What are the three Full Screen Display settings for?

Basically the three Full Screen modes each try to achieve a full screen image in different ways: The first one asks your computer for a resolution appropriate for the game - for instance, 1280x720. Every driver handles that in different ways - some will give the game what it asked for, others will stretch, shrink, letterbox or otherwise work out how to display the game based on their own logic. This means depending on your set-up the game can look very different. The other two scale the screen to match your current desktop resolution, one preserving the aspect ratio whilst the other simply stretches. For the HD assets, if the game detects you've got a resolution of 1920 x 1080 or higher it will use these graphics, otherwise it will use the normal set.