Almost 100% guaranteed, but it is still possible for the OS to be delayed in sending it the coordinates. Of course that means that the OS is badly written.

By these standards, the only well-written operating systems I've ever seen are BeOS and QNX/Photon. Probably iOS (at least, older versions of it) deserves to be on the list, despite mouse use being atypical.

Which... yeah, that actually might be true.

iOS is the odd one out, because user input has first priority. IIRC, it cannot even do network I/O if you are holding your finger on the screen; all interrupts except those from the touchscreen are disabled for the duration.

I can say for sure this is true on sway (wayland) because while messing around seeing if the proprietary nvidia driver finally works yet (when will I learn... for anyone curious nvidia is still nvidia) one of the things I had to do was manually disable hardware cursors.