[tbb-bugs] #26607 [Applications/Tor Browser]: verify that subpixel accuracy of window scroll properties does not add fingerprinting risk

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#26607: verify that subpixel accuracy of window scroll properties does not add
fingerprinting risk
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 Reporter:  mcs                                  |          Owner:  tbb-
                                                 |  team
     Type:  defect                               |         Status:  new
 Priority:  High                                 |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tbb-fingerprinting, ff60-esr,        |  Actual Points:
  TorBrowserTeam201904                           |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by Thorin):

 > We think this means "half pixels on a macOS Retina or other high
 resolution display."

 See [1], you need to scroll up and down for a tiny bit. Tests records up
 to 100 scrollX values - as it gets a value, it checks if it is an integer
 - if it is, it continues, if not, then it stops the test. Last value is
 display and logic for sub-pixel detection (yes/no)

 On my ancient Android 6, I get half pixels. If I zoom it and refresh I get
 13 decimal places. I don't have a Mac or retina display.

 I suggest we file an issue for Tor Uplift to round sub-pixels to an
 integer when RFP = true

 [1] https://thorin-oakenpants.github.io/testing/
 - Note: if the test vanishes from here, it's because I made it a one liner
 and moved it to
 https://ghacksuserjs.github.io/TorZillaPrint/TorZillaPrint.html

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26607#comment:13>
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