Plasma Breeze Mozilla / Browser render problem: Gaps between app and x-container border

Between the app and x-container there is a gap which result in borders being drawn outside of the app window. This happens on Plasma 5 & 6 for flatpak and pacman apps from Mozilla and Brave (there might be more apps) on X11. Does anyone had the same problem and solved it? Thank you for any hint! **Edit:** When maximized the gaps are gone **Edit 2:** The problem appears also when using Brave