Sometimes in Windows 10 Chrome or Firefox, whenever you try to play a full-screen Youtube video, the taskbar still shows and does not auto-hide. Here’s how you can fix it;
Close all browsers and press Ctrl+Alt+Del keyboard buttons simultaneously to bring up Task Manager window. Click “Task Manager” from the menu;
In the next window, right-click “Windows Explorer” in the Processes tab, and select “Restart” option;
Next, play Youtube video in full screen in Chrome or Firefox to verify if the issued is fixed. If not, then you can go to Chrome or Firefox browser forums and post your specific issue there.