So when going fullscreen, opera will pick the monitor it first fullscreened on. I have it set so opera gx opens on my second monitor, which is where I want to use it usually. If I full screen anything, youtube, the browser itself, it’ll fullscreen on the first monitor you fullscreen that window on. However, after you fullscreen a window for the first time on a display, then it’ll only fullscreen on that monitor. If I fullscreen on my primary and then move the window to my second and fullscreen, the fullscreen will appear on my primary still even though the window can only be controlled on the secondary. This persists until the window is closed and opened again, at which point it happens again whenever you fullscreen on a monitor again. The window just selects whatever display it fullscreens on first and then only fullscreens on there until it’s closed. It's really annoying to close the entire browser and then start it up again just to fullscreen on my main monitor.