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Letterboxing and Window Fingerprint

Why Tor Browser draws grey bars around the page, why maximizing can make you unique, and why extra extensions destroy the shared fingerprint.

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Letterboxing gray bars in Tor Browser A Tor Browser window with gray letterbox bands around a smaller content pane. Sites see the inner size bucket, not the exact window. Leave the bars; do not add extensions to hide them. Tor Browser tab https://example.invalid Sites see the inner pane — not this window size 01 Leave the bars Stay in the size bucket 02 No extra add-ons Extensions make you unique Tor Browser tab https://example.invalid Sites see the inner pane 01 Leave the bars Stay in the size bucket 02 No extra add-ons Extensions make you unique
Gray bands are letterboxing: the page is rounded into a shared size bucket so your exact window is harder to fingerprint. Leave the bars. Do not install extensions to hide them.

Short answer: The grey bands around the page are letterboxing. Tor Browser rounds the content window to a shared size (multiples of 200×100 CSS pixels) so your exact monitor is harder to use as a fingerprint. Do not install random extensions to “fix” the bars. Do not maximize without understanding that you are leaving the bucket.

The fingerprinting glossary is the one-line definition. This page is the habit.

What the bars are

The Tor Project’s fingerprinting page: the browser starts with a content window rounded to 200×100 steps so users land in a few size buckets. Resize, maximize, or go fullscreen and that trick fails — so letterboxing adds margins. The window can look close to the size you asked for; websites still see a bucket, not your raw geometry.

Older Support text called those margins “white.” In the dark theme they read as grey. Same feature.

What to do

  1. Leave the grey bands. They are the cover. Sites should see the inner pane size, not your exact window.
  2. Prefer the default window size the browser opened with. If you must go large, keep the bars — maximizing without letterboxing is how you leave the default bucket.
  3. Do not install extensions to hide the bars, “block more trackers,” or theme the UI. Privacy Guides and the Tor plugins page agree: extra add-ons and about:config edits make you stand out. Only bundled add-ons are tested. Security-level presets are the supported way to cut features: security levels.
  4. Do not spoof another OS in the User-Agent. Tor Browser already normalizes User-Agent per platform and does not let you pick a fake OS. A choice would shrink the crowd.

What letterboxing does not do

It does not hide your IP — that is the circuit. It does not hide a login. It does not match desktop protections on iOS WebKit: Orbot vs Tor Browser Android vs Onion Browser.

Leave the window where the browser put it unless you have a reason. Rare is trackable.

Sources

See also: New Identity vs New Circuit, Orbot vs Tor Browser Android.