Hello @Seralyne, we're using one issue report per unofficially supported game title, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/8760#issuecomment-3036739556.
Ah yeah I wasn't entirely sure whether this would be considered a separate issue, as the other issues seemed to pertain to the "The War Within" (or "Retail") version of the game, while my issue is specifically with the Mists of Pandaria Classic version of the game, and figured the worst case that could happen was it being moved.
Compatibility Report
System Information
I confirm:
Symptoms
When the window is defocused for an amount of time, it will hang all display updates. When refocused, it will attempt to catch up. The longer it waits, the higher the chance of failing. I can only defocus the window for around 10 seconds before it is irrecoverable.
Reproduction