When VSync is enabled, and Max Framerate is set to unlimited, mouse movement becomes jittery. When VSync is disabled and Max Framerate is set to 60fps, mouse movement is regular.

This is noticeable while looking around, and dragging items in an inventory.

Steps to reproduce:

In Options > Video Settings, enable VSync, set Max Framerate to Unlimited. Notice mouse movement is jittery.

Disable VSync, then set Max Framerate to any value, and notice mouse movement is normal.