Playing League of Legends on an external monitor with Apple silicon (display glitchiness)

I just bought myself a 14" MacBook Pro (M1 Pro), which I use as my personal computer, including for casually playing League of Legends (a popular MOBA game).

I am connected to an external display (LG UltraFine 4K 24MD4KL-B) and some external peripherals (mouse, keyboard).

League of Legends runs via Rosetta 2 on Apple Silicon macs, and runs perfectly fine on the built-in display. I run into some issues, however, when connecting my external display: input lag, screen stutter/flaky pixels/glitchy display (I can provide a video to illustrate).

The weird thing is, disconnecting and reconnecting the thunderbolt cable once in-game solves the issue: display becomes pristine, there's no discernible input lag, everything works perfectly. However, this workaround causes the OS to malfunction post-game: application switching no longer works, and subsequent games load into a black screen. A restart solves the issue (but it means I have to restart my machine after every game).

This is kind of a frustrating issue, so I wonder if anyone has faced a similar problem or knows of a potential fix? Also, I don't know which part of the chain is to blame in this: the game itself? macOS Monterey? Rosetta 2? I want to push for an investigation, but I don't want to go barking up the wrong tree.

Full specs:

• 2021 14" MacBook Pro (M1 Pro, 16GB, 1TB)
• LG UltraFine 4K 24MD4KL-B
• External peripherals
• macOS Monterey 12.0.1 (21A559)

Thanks in advance for any help!

Solution 1:

Welp, I've found the solution by myself!

For anyone who runs into this issue in the future: you have to run the game in Borderless mode (Options > Video > Window mode).

Game runs smooth as butter now :)