TorBrowser FAQ: "Your activity could be linked to other browser by moving your mouse from one browser into the other". How does this work?

From the [Tor Browser FAQs](https://support.torproject.org/tbb/tbb-17/):

> However, be aware that when using Tor and another browser at the same time, your Tor activity could be linked to your non-Tor (real) IP from the other browser, simply by moving your mouse from one browser into the other.

What? So, moving the mouse cursor from one window to another is trackable? Is this general for all browsers? How does this even work???? How much information am I giving away when this happens?

aware browser browsers general mouse moving non privacy tor