If you play 1v1 LOL every day, consider using a dedicated desktop application that strips away the browser entirely.
How to use Alt+Enter in a browser? Normally, Alt+Enter doesn't work in a browser tab. However, if you download the game via Steam (Yes, 1v1 LOL is on Steam), pressing Alt+Enter will toggle true fullscreen instantly.
Inside the settings menu, look for a toggle labeled: 1v1lol work fullscreen
Set this to "Fullscreen" or "Borderless Fullscreen." Borderless is often better for 1v1.LOL because it allows you to alt-tab without crashing the game window.
Before we fix the problem, it helps to understand why the issue exists. Unlike a downloaded PC game (like Valorant or CS:GO), 1v1 LOL runs inside a web browser using WebGL and HTML5. Browsers are designed to protect your system, which means they often restrict "true" fullscreen (API fullscreen) unless you physically click a button on the screen. If you play 1v1 LOL every day, consider
Furthermore, many schools and offices block the requestFullscreen() JavaScript command. When you click the fullscreen icon inside the game and nothing happens, that is likely an IT policy blocking it.
Common culprits:
In fullscreen, mouse acceleration ruins muscle memory. Go to Windows Settings > Bluetooth & devices > Mouse > Additional mouse options > Pointer Options > Uncheck "Enhance pointer precision."