I have a 4k screen on my laptop, and found that playing Minecraft at the full 4k resolution was laggy, so I lowered the resolution. When I went into a world, however, I found that almost any time I stopped moving my mouse, the crosshair would pick a seemingly random direction and drift that way until I scrolled between items, began moving the mouse, or clicked either mouse button. I tried turning off my USB mouse and using the trackpad, but the issue persisted.