I have an addon that dynamically adjust the RenderScale CVar every 5 seconds depending on user FPS. It was working great until the most recent update. I am able to replicate this issue. If you have the Character Panel open, your bags open, and then open/close the map, the game hangs and the gxEngine restarts (before do this, change the render scale). There are not driver crashes or errors in the event log, or addon errors.
UPDATE:
So, I found another CVar called dynamicRenderScale, that is is BETA. I don’t know when this was added, my added was working fine post expansion launch. But I suspect that Blizzard is actively making changes to how the RenderScale is applied, and this is causing my issues. The note mentions that it may cause hitching, which is exactly what I see. Hopefully this gets fixed soon.
EDIT:
You can replicate this without even using the WoW API, just change the render scale slider to any value, then open you bags, character window, then map, and the game will hitch. Even after setting your CVars back to default.