If you experiencing strange mouse movements, and you feel that "something is off" with your sensitivity; turn off "Fullscreen Optimizations" in the Compatibility Tab of the csgo.exe properties. : r/GlobalOffensive
Fix The Steam Deck Mouse Bug In CSGO And Apex Legends – QM Games
Solved: What is this strange box near my mouse cursor? How... - Adobe Support Community - 10633133
FIX: Mouse Cursor Moving on its own (Solved) • Repair Windows™