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Tor Browser is using letterboxing to prevent fingerprinting by screen size.

Now, I have done a test on Cover Your Tracks website from EFF and the results show a unique fingerprint and the reason for that is a Window Size of 1099x999. Shouldn't it be 1100x1000 instead?

Edit: My OS did not do the window resize "to fit on screen", and even if it did, the letterboxing should put black borders on the sides so websites can not see the screen size.

Edit 2: It is issue 40081, i am using a high dpi screen. Thank you Peter Gerber for answering!

Screen size fingerprint result

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The reason here is obvious: your OS did the window resize "to fit on screen"

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    @Alexay how did you arrive at this conclusion?
    – Peter Gerber
    Commented Jul 26, 2021 at 19:34
  • The OS'es do use a GUI managers to display every graphical application content, so unless you'll set the size of the window's viewport at the moment of API call and force it to be preserved - it's a normal GUI behavior to auto-resize windows: Gnome, KDE, etc... From version to version it used to change a bit, so - to drop this specifics it was a wise move to set the explicit and immutable window size, so any browser's component - including add-ons - will be sandboxed for sure
    – Alexey Vesnin ♦
    Commented Jul 31, 2021 at 7:25
  • @AlexeyVesnin perhaps we could add the comment above into the answer if that is good with you?
    – Swangie
    Commented Aug 4, 2021 at 15:06
  • @Swangie the answer here can't be any longer because this is just it as it is.
    – Alexey Vesnin ♦
    Commented Aug 6, 2021 at 16:27
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Yes, correct, it should be some nicely round number.

Sadly, there are still some bugs in the implementation. The ones I've observed myself are:

I could not find any bugs indicating that fitting a window to the screen could cause this, as suggested by Alexey. If that is what you observe, please report it as bug.

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