External monitor detection test
Checking…
Reading what the operating system reports about attached displays.
This page runs, in your own browser, the same display check a remote interview or online proctoring page can run. Nothing is uploaded — every reading below stays in this tab.
Desktop map — drawn to scale
Screens are placed at their real coordinates. Mirrored screens land on the same spot, so they stack.
Signals
| # | Label | Position | Size | Available | Scale | Depth | Role | Bounds |
|---|
What this test reads
Chromium browsers expose two things about your displays: screen.isExtended, a single true or false that needs no permission, and getScreenDetails(), which returns the position, size, scale and label of every attached screen once you grant the Window Management permission.
From those readings the verdict above separates three situations: one display, several displays extending the desktop, and several displays showing the same picture.
The map and the table show the raw values, so you can see exactly what any other site would see about your setup.
Before a remote interview
If a company asks you to sit an interview or an assessment on a single screen, check your setup here first rather than finding out mid-call.
- Open this page in the browser you will actually use — a result in Chrome says nothing about Firefox.
- Grant the permission once so the full layout is readable, then read the verdict.
- If it says Extended, unplug the second monitor, or close the laptop lid when you are working on an external display.
- Watch the verdict change to Single display. The page updates live, so you can confirm it as you unplug — no reload needed.
- Remember what counts: a docking station, a USB-C hub, a tablet used as a sidecar display and a TV left connected over HDMI are all extra displays.
None of this makes a proctoring system approve you. It only tells you what that system will read.
What a page can and cannot learn
Without permission a site learns one bit: whether your desktop spans more than one screen. It does not learn how many there are, how large they are, or what they are.
With the Window Management permission granted it also reads each screen's pixel size, its position in the virtual desktop, the scale factor, the colour depth, whether the screen is built-in or primary, and on some systems a manufacturer label.
No API exposes what is on those screens. A page cannot see your windows, your other tabs or the contents of a second monitor. Screen sharing is a separate permission and always shows a visible indicator.
Chrome remembers the choice per site. You can revoke it from the icon at the left of the address bar.
How the verdict is reached
Extended — two or more screens are reported and each sits at its own position in the desktop coordinate space.
Mirrored — two or more screens are reported at an identical position and size. Nothing else produces that; extended screens never overlap exactly.
Mixed — three or more screens where one pair duplicates each other while another extends the desktop.
What this cannot see
Mirroring is only detectable when the operating system hands the browser each panel separately. macOS merges a mirrored set into a single logical screen before Chrome ever sees it, so a mirrored MacBook reports as one display.
When the verdict is Single display on macOS, read it as “one display or a mirrored set” — no browser API can separate the two. Windows and most Linux setups do report duplicated panels, and those are caught here.
A capture card, a video splitter or an HDMI dummy plug placed after the graphics card is invisible to every browser API. Virtual displays created by remote-desktop or tablet-as-display software usually do appear, as ordinary extra screens.
Requirements
Detail beyond the screen count needs the Window Management permission, a Chromium browser version 100 or newer, and a secure origin — https:// or http://localhost.
Firefox and Safari implement neither reading, so the verdict there is Not supported rather than a display count. A proctoring page cannot run this check in those browsers either.
Everything updates live: plug a monitor in, switch to mirroring, or drag this window across screens and the page follows.
Frequently asked questions
Can a website detect if I have two monitors?
Yes, in Chromium browsers. screen.isExtended reports whether the desktop spans more than one display, and it needs no permission at all. Anything more detailed — how many screens, their sizes and their positions — requires the Window Management permission, which you have to approve.
Can an interview or proctoring site see my second screen without asking?
It can see that one exists, but nothing about it. The count, the sizes and the positions stay hidden until you approve the permission prompt. What is displayed on any screen is never readable without a separate screen-sharing permission, which shows its own indicator.
Does closing the laptop lid remove the built-in display?
On most laptops, running with the lid closed and an external monitor connected leaves exactly one active display, and this test then reports Single display. Confirm it here rather than assuming — some docks keep the internal panel awake.
Why does mirroring show up as a single display on my Mac?
macOS combines a mirrored set into one logical screen before the browser is told about it. Chrome receives one screen and reports one. Windows and most Linux desktops report the duplicated panels separately, and this page catches those.
Does external monitor detection work in Firefox or Safari?
No. Neither screen.isExtended nor getScreenDetails() is implemented there, so this page reports Not supported. The same limitation applies to any site trying to run the check.
Is any of this sent to a server?
No. The page is static, has no analytics and no cookies, and makes no network requests after it loads. Every value shown is computed inside your tab.
Will unplugging a monitor mid-session be noticed?
The browser fires a screenschange event, so any page holding the permission sees the layout change the moment it happens. This page demonstrates that: unplug a display and the verdict updates without a reload.
What counts as an external monitor?
Anything the operating system treats as a separate display: an HDMI or DisplayPort monitor, an output from a USB-C dock, a TV, a projector, a tablet used as a second screen through Sidecar or a similar app, and virtual displays created by remote-desktop software.