Solace от incognizant

Solace locks your browser to the tabs you choose — one tab or all of them. Set a timer or go indefinite. New tabs, tab switching, and off-site navigation are blocked until your session ends.

An eclipse animation in your toolbar tracks your progress. When time's up, keep going or take a break. Ctrl+Alt+S starts or stops a session from anywhere.

No accounts, no blocklists, no data collection. Just a timer and a lock.