Update: what worked for me (AMD on Windows10):
- Right click on desktop -> Display settings
- Scroll down -> Graphics settings
- Select “classic app” in the drop down and browse for the BlizzardBrowser (NOT the WC3.exe) and add it to the list. Default path is C:\ Program Files (x86)\Warcraft III Beta\x86_64\BlizzardBrowser\BlizzardBrowser.exe
- Select Options for BlizzBrowser and set to “High performance”.
- Restart the Battle Net app.
??? - Success.