Opened 2 years ago

Last modified 2 years ago

#21341 new defect

Screen size not rounding if Windows not at default DPI_TBB 6.5

Reported by: joebt Owned by: tbb-team
Priority: Medium Milestone:
Component: Applications/Tor Browser Version:
Severity: Normal Keywords: tbb-6.5-regression, tbb-fingerprinting-resolution
Cc: arthuredelstein Actual Points:
Parent ID: Points:
Reviewer: Sponsor:


TBB 6.5 Win: Automatic screen size rounding.
Note: the 1st reported screen sizes for v6.5 are with Windows' DPI = 110 (115% of default 96 DPI).
Clean TBB install in new folder, with default prefs. No extra addons.
Reported screen size 993x695 on the actual 1920x1080, 16:9, 21.5" diagonal monitor.

In v6.5, toggling "extensions.torbutton.resize_windows" to True (default=false), restarting TBB - reports 825x600 screen size (& actual screen is quite small). Not using available width or height.

If Windows DPI = 96, TBB 6.5 rounds screen to 1000x900.
But... v6.08 rounded correctly whether Windows was 110 or 96 DPI (see details).
I see no reason why TBB can't correctly round if the system DPI is changed (it worked in v6.08). Presumably, it rounds correctly for many different screen sizes, resolutions - but can't handle changing DPI from 96 to 110?

  • In TBB 6.08, Windows @ non-default 110 DPI, changing "extensions.torbutton.resize_windows" to *true,* rounded screens to 1000x800 on the monitor above. With that pref at default *False,* the height wasn't rounded correctly (gave 728 - 729), but width was still 1000.
  • In v6.08, if Windows' DPI was 96, & "extensions.torbutton.resize_windows" = true, it rounded screen to 1000x900 (instead of 1000x800 @ DPI=110). More pixel height available, but it still rounded to a multiple of 100.
  • Is it still true that: changing the browser's default font or size,

or changing minimum font size from "None,"
or unchecking "Allow sites to use their own fonts..." can be detected for fingerprinting?
If yes, @ default 96 DPI the above 3 settings often make some sites too small for many users, especially larger text areas. I think much of the problem has to do with font scaling in Firefox.

If changing the above three settings is NOT detectable, then choosing another font can increase text size considerably without changing anything else.

++ The site displays a lot of monitor data. Whether accurate or not, some are strange values that presumably remain constant for a user and may be identified at multiple sites.
TBB 6.05, shows 22" diagonal and display area 16.35" × 14.72" - an unusual monitor size.

Other data it shows (when Windows' DPI was 96):
Monitor data
Horizontal resolution: 1000 pixels
Vertical resolution: 900 pixels
Diagonal: 22 inches (55.88cm)

Aspect ratio: 1.11:1

Display size: 16.35" × 14.72" = 240.66in² (41.54cm × 37.38cm = 1552.66cm²) at 61.15 PPI, 0.4154mm dot pitch, 3740 PPI²

How common is an Aspect ratio of 1.11 : 1, and display size 16.35 x 14.72, unless you're a Tor Browser user?

Child Tickets

Change History (3)

comment:1 Changed 2 years ago by cypherpunks

Component: - Select a componentApplications/Tor Browser
Owner: set to tbb-team

comment:2 Changed 2 years ago by gk

Keywords: tbb-6.5-regression tbb-fingerprinting-resolution added

comment:3 Changed 2 years ago by arthuredelstein

Cc: arthuredelstein added
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