Your profile gives itself away by window size first

Any page reads the screen your browser claims and the window it is looking through. Both are ordinary values, and a site's script needs no permission for either.

On a page that declares a viewport, a window wider than the screen describes a machine that does not exist. Without that line a real phone lays out wider than its screen too, so a site only draws its conclusion after it.

1. 1

   Asks for the screen

   Width and height of the display the browser reports.

2. 2

   Asks for the window

   Width and height of the area the page is drawn in.

3. 3

   Compares them

   A window larger than the display is a contradiction inside one browser.

Your profile takes its screen from its seed, and across 900 desktop profiles of the current fingerprint generation the pool looks like this.

Eighteen distinct screens appear, from 1280 by 720 to 3840 by 2160. That spread works for you: a pool of one size would be a signal of its own.

You work on the display you own, and a common one is 1920 by 1080. Maximise a window there and it fills that size regardless of what the profile inside it claims.

So the profiles that disagree are the ones whose screen is smaller than the display in front of you. On a larger display there are more of them, on a smaller one fewer.

On a phone profile the gap is impossible to hide: a handset claims 360 points across, and a window stretched by hand would be three times that.

That is why a phone profile opens at the size of the handset, and a remembered size is not restored. The desktop case is the same contradiction with smaller numbers.

Your window opens no wider than the screen the profile claims. A smaller window stays small: the claimed screen is a ceiling, not a requirement.

The ceiling sits on the outer size of the window, and a page reads the inner area, always smaller. The margin works in your favour.

1. 1

   One rule for every window

   Every window of a profile opens by the same rule.

2. 2

   The claim is the ceiling

   Width and height never go above the screen the profile reports.

3. 3

   A phone is exact

   A handset window equals the handset screen rather than merely fitting inside it.

The app carries the Fingerprint self-test (profile menu > Test fingerprint): it opens a page and checks line by line that the browser answers what it was told to. The line about the window is in there too.

On a desktop profile that line does not run. It is grouped with the phone ones, next to touch points and screen orientation, and on a desktop fingerprint the whole group is skipped.

So a person sees a report where everything matched and learns nothing about the gap. Until that line moves to the common group, the window is something to check by eye.

Advice does not work here: window size is something people drag with a mouse every day without thinking about numbers. The gap appears from one pull towards the edge of the screen.

So a profile window does not open wider than the screen it claims. Smaller is fine, wider is not: the claimed screen is a ceiling and it holds by itself.

Two profiles on one monitor each get their own claimed screen, and usually a different one: across two thousand pairs they matched in twelve cases out of a hundred. Different widths of maximised windows are not a fault but the very thing profiles exist for.

Equal widths on two windows would tell a platform more than unequal ones. Matching sizes read as one machine rather than two.

A maximised window takes the width of your monitor, while the screen a profile claims has nothing to do with it. So the ones that disagree are the profiles whose claimed screen is narrower than yours.

Across nine hundred draws it looks like this: on a 1366 monitor roughly one profile in twelve disagrees, on 1920 six in ten, on 2560 almost eight, and on 3840 nearly all of them.

The conclusion is plain: a person with a large monitor is more exposed while doing nothing unusual.

The ceiling applies when a window opens, so older profiles are covered as well: their remembered size is trimmed to the claimed screen on the next launch.

Nothing needs moving by hand. If a window was already smaller than the claimed screen, it stays that way: the ceiling never stretches anything.

A profile window opens eight hundred and twenty points tall. On the 1280 by 720 and 1280 by 800 screens the claimed height is smaller than that.

So on those profiles the window is taller than the claimed screen before anyone drags it. The ceiling trims height as well, not only width.

The temptation is there: give every profile 3840 by 2160 and the window never runs past the edge. That would turn the set of screens into a marker of its own.

Eighteen kinds with uneven shares look like what a platform sees among ordinary people. One size for everyone would be spotted sooner than the gap.

Every share in this entry was counted the same way: take nine hundred profiles of the three desktop kinds on the current fingerprint generation, read the claimed screen width of each, and count how many are narrower than the chosen monitor.

You can repeat it: the numbers come out of the same code that builds a profile's fingerprint rather than from a table written by hand. We recounted with three different sets of profile names, and the share came out between fifty-nine and sixty-three percent on a 1920 monitor.

That is why the text says about six in ten rather than one exact figure. An exact figure here would mean one sample passed off as a property, and we have been there.

Open any browser console and read the screen and the window side by side. The comparison takes one line and needs nothing installed.

Do it in a maximised window and in a small one. A profile that answers honestly gives a window no larger than the screen in both.