[Client] Screen covered in dark blue pattern

This has been happening to me for the last while (I don't know exactly how long, but the last ~6-10 months), where, when I get into the loading screen, the screen gets covered with some dark blue pattern. Sometimes it covers the whole screen constantly, and other times it flashes across parts of the screen. It seems to happen when I alt+tab out of the client when it starts loading, but it doesn't happen consistently, as I do this most games, and this bug only occurs rarely. This might be a hardware issue, but it only happens with League and not any other games or software. Usually this has not been a problem, as I manage to fix it in the loading screen or early into the game by spamming alt+tab, but today it happened again, and it didn't go away until after I restarted the game. It's also not a purely graphical bug; when it covers the full screen, I can't hear anything from the game either. I don't have any picture or videos of this happening, unfortunately.

Edit: Happened again, this restarting didn't fix it. I tried to get screenshots, but it cleared up every time I did. Other than those times, the bug was constant. At this stage, I'm thinking it's probably just my computer, but this has still only ever happened with League, and now I'm kinda afraid to try playing again.

(Should this be under Client? I'm not sure.)