If you Alt-Tab out of the game and back, or save/reload, does that get the game screen to update?
When you tried windowed display mode, did you also try dropping the resolution and graphics settings?
If you start the game on the internal graphics, do you have the same problem?
Please email supportdos2@larian.com with a save when this happens (or reply/PM with a Dropbox or Google drive link, etc), as well as the rest of your system specs (you can use the 'System Report' button from the 'About This Mac' Apple menu item, or the 'System Information' app in the utilities folder), including the OSX version number and model identifier. If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file from your profile folder.
Each save is a folder in the '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/PlayerProfiles/<ProfileName>/Savegames/Story' folder.
In Finder you can zip a folder by right clicking and selecting compress.
Also include the sampler output from the Activity Monitor:
1. verify local files (right click on the game in the Steam library and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button).
2. launch Activity Monitor
3. start the game and trigger the largest explosion that is handy
4. Alt-Tab back to Desktop (windowed display mode may help) and check Activity Monitor. What do you see? How much CPU does the game consume?
5. double click on the game process and press the 'Sample' button. Wait until Sampler is finished
6. save sample output using 'Save' button
