I'm also having this problem. Before, I could open Opera GX and the browser would open while the windows taskbar is still visible at the bottom. Now, the taskbar is hidden and I can't find a way to make it visible again. I tried pressing F11 to go to fullscreen mode and then pressed it again to get out of fullscreen mode. When I do that, I can see the windows taskbar but it hides part of the page, plus it is not a good long-term solution as I have to do this every time I open up Opera GX.