Hello @maboleth, we're using one issue report per unofficially supported game title, and usually I'd transfer this issue report to #556, however, this has also been reported as a general issue with launchers at #3495.
Closing as a duplicate of #3495.
Latest proton 5.0.1 here.
ESO compatibility seems flawless, however since upgrading proton to 5.0, launcher has fake window borders, that shouldn't be there. On Proton 4.11.12 everything is fine.
Take a look at the screenshot with latest proton:
