Old School RuneScape (and by extension, RuneLite) doesn’t have a built-in full screen setting.
However, there is a plugin that lets you play in either Fullscreen Exclusive Fullscreen Borderless and I have been using it for a while without any issues.
- Click the Settings icon in the top right of RuneLite.
- Click the Plugin Hub icon.
- Search for Fullscreen and click Install. You may receive a warning about it not working properly on your version of Windows but you can safely ignore this.
- You will also need to ensure ‘Enable custom window chrome’ is disabled: Plugins > RuneLite > Window Settings > Enable custom window chrome (uncheck this).
- Go back to the main settings page and select either Exclusive or Borderless.
- Restart RuneLite. This is necessary to get rid of the title bar.
You can now play RuneScape in fullscreen by clicking the icon in the bottom right corner for a more immersive experience.
I can confirm that this works fine when using multiple monitors and on a 2560×1440 240Hz monitor.