Left Mouse Button Click moves Cursor to the top edge of the Screen

Dear bug-collecting community,

I am not entirely sure whether this is the correct place to post but some titles are about strange mouse behaviours, so I'll give it a try!

Whenever I am in game and use the left mouse button to click while having slight movement with the cursor, the cursor immediatly jumps to the top of the screen and starts to move the screen. It does not happen on every click, but in most of the cases and makes the game unplayable - you never see your character. It first showed in Ranked Queue (great experience) and I can reproduce it in any other game mode I tried (Normal, ARAM, Practise Tool). Outside of league, my mouse behaves like it should. Clicking frantically while moving it does not provoce any other reaction than it should.

I have tampered with the imput.ini / persisted settings file a bit to move "AttackMoveClick" to the left mouse button. However, it worked like that for the past three years and I haven't changed anything in the file recently. Also, I edited it differently to have no more function on the left mouse button and after testing that reset everything back to default and it still moves the cursor to the top whenever I use the left mouse button, regardless of its current function.

I play on a Mac and the bug showed both under the old High Sierra and persisted after my update yesterday to Mojave.

Currently, I use an Evoluent Vertical Mouse C (ugly as the night but helps with wrist issues) without the driver. I have no other mouse at hand to compare it, but the same bug occurs whenever I frantically click with the trackpad so it seems to be unrelated to the mouse itself.

If anyone has an idea what might cause that jump and how to resolve it, I would be very thankful!

Best regards