Someone spying on someone else

Special report Computer science boffins have demonstrated a side-channel attack technique that bypasses recently-introduced privacy defenses, and makes even the Tor browser subject to tracking. The result: it is possible for malicious JavaScript in one web browser tab to spy on other open tabs, and work out which websites you’re visiting.

https://www.theregister.co.uk/2018/11/21/unmasking_browsers_side_channels/