RunTheTests
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Multi-Monitor Detector

See every display attached to this machine, with the resolution and scaling of each.

Result

This browser cannot enumerate displays

This browser has no way to enumerate displays. Only Chromium-based browsers implement the Window Management API, so what you can see here is the single screen this window occupies — which is not evidence that it is the only one attached.

Diagnostic telemetry
DisplaysThis page can only see the screen it is on
Not enumerated

What this cannot tell you

  • A web page cannot see displays other than the one holding the window without permission. Until it is granted, this tool reports that it does not know rather than guessing — a page claiming one screen without asking is describing its own window, not your desk.
  • Only Chromium-based browsers implement the Window Management API. Firefox and Safari cannot enumerate displays at all.
  • Reports what the operating system exposes. Physical size, panel model, refresh rate and the arrangement of displays in space are not part of it.
  • Nothing about your displays is sent anywhere or stored. The result is rendered in the page and is gone when you leave.

Take this with you

How this test works

The Window Management API can list every display attached to the machine, but only after you allow it — so this tool asks, and until you answer it says plainly that it cannot tell a single-monitor desk from a three-monitor one. That refusal is the point: `window.screen`, which needs no permission, describes only the display this window happens to be on, so a detector built on it reports "one screen" with complete confidence on a three-screen setup. Once permission is granted the tool lists each display with its resolution, scale factor, colour depth and whether it is built in, and it re-reads the list when a display is plugged in or removed.

What the results mean

Displays
How many screens the operating system reports. Shown as 'not enumerated' rather than as 1 before permission is granted, because those are different claims.
Scale factor
The display scaling the OS applies, as a multiplier. 1× is unscaled; 1.5× and 2× are typical on high-density panels.
Mixed scale factors
Two or more displays running at different scaling. The usual cause of a window turning blurry when dragged between screens.
Built in / external
Whether the OS considers the display part of the machine. Laptops report their own panel as internal.

Common problems and fixes

It says it cannot enumerate displays
Either the browser is not Chromium-based — Firefox and Safari have no such API — or the permission was refused. The permission can be re-enabled from the icon at the left of the address bar.
Windows go blurry when I drag them between screens
That is the mixed scale factor this tool flags. The window is re-rasterised for the new density, and applications that do not redraw cleanly stay blurry until restarted on that screen. It is a settings mismatch, not a fault.
A monitor is attached but not listed
Check it is actually receiving a signal rather than only power. A display asleep or on the wrong input is not reported by the operating system at all.

Frequently asked questions

Why does the page need permission to see my monitors?

Because the number, size and arrangement of your displays is a strong fingerprinting signal — it is unusual enough to help identify a browser across sites. Chromium put it behind a prompt for that reason, and a page that reports your screens without asking is not doing what it claims.

Why do other sites detect my monitors without asking?

They almost certainly do not. What they can read without permission is the screen the window is on, so they report one display and are wrong whenever there is more than one.

Does this work in Firefox or Safari?

No. Neither implements the Window Management API, so neither can enumerate displays. This tool says so rather than falling back to a guess.

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