Ok, this only happens when OceanPostProcess is used and with version 4.1.0. You can see the bug happening here: Laje de Santos em Realidade Virtual (resize the browser window to see). Exact same code is at Babylon.js Playground but using 4.0.3 it does not reproduce.
Using FF73.0.1 and re-sizing window to < 0.5 of full size the blurring of the island becomes noticeable, the smaller it goes the more blurred it gets. In some ways I would expect this in real life. However switching FF to responsive mode in portrait also make the island blurred, not as bad in landscape.
But this is related to any PostProcess effect then. I couldn’t find anything in the docs that allows one to change the width/height after the constructor PostProcess - Babylon.js Documentation. Am I missing something or is there no way to do it currently?