A frameless window covering its screen is in fullscreen display mode
A frameless window that covers its screen matches (display-mode: fullscreen). N/A for windows with a frame, installed web apps, mobile claims, Brave and non-Chromium engines (severity: warn).
What a detector infers
A desktop browser window has a frame — tab strip, toolbar, borders — so its outer size is larger than its viewport. The one state without a frame is fullscreen, and frameless-window-vs-fullscreen accepts a frameless window as long as it covers its screen from the origin, on the reasoning that such a window must be fullscreen. This row checks that reasoning instead of assuming it.
A window that really is fullscreen says so: F11 and element fullscreen set the window's fullscreen state, and the display-mode media query follows it to 'fullscreen'. Viewport emulation produces the same geometry without the state. Measured: Playwright's viewport emulation (1280×720 at a device scale factor of 2) and chrome-headless-shell both report a frameless window exactly covering its screen while display-mode says 'browser'.
Genuine windowed browsers never take that shape, and genuinely fullscreen ones report 'fullscreen'. Installed web apps report 'standalone' and are N/A, and so are mobile claims, non-Chromium engines and Brave, which varies window geometry as a privacy feature.
How to resolve it
Let the viewport be the real window instead of emulating one — in Playwright, viewport: null (no_viewport=True in Python) — and run headed, or give the window a size that leaves room for the browser's own frame.
Nearby checks in Screen & display
- A desktop browser does not expose touch-event feature detection
desktop-has-no-touch-eventsWhether 'ontouchstart' in window is true is not decided by the hardware. - An element that went fullscreen gets the screen and the fullscreen display mode
fullscreen-is-real-fullscreenWhen a page calls requestFullscreen() during a click, Chrome normally makes the whole window fullscreen: document.fullscreenElement is set… - devicePixelRatio is the scale layout actually rasterises at
dpr-vs-device-pixelsdevicePixelRatio is meant to be the scale at which layout is rasterised, and several parts of the browser act on the real scale rather than… - Pointer coordinates agree with the window's own position
event-coords-vs-window-geometryA pointer event carries viewport coordinates (clientX/Y) and screen coordinates (screenX/Y), and the browser derives the second from the… - screen.width/height backed by the display (matchMedia)
screen-matchmediaThis check reads window.screen.width/height, then asks the CSS media-query engine the same question via matchMedia("(device-width: Npx) and… - devicePixelRatio matches matchMedia resolution
dpr-matchmediaThe check compares window.devicePixelRatio against the layout engine's own answer, querying matchMedia with a min-resolution/max-resolution… - The screen times devicePixelRatio is a panel a monitor actually has
dpr-screen-vs-panelOn Windows, screen.width and screen.height are reported in scaled pixels — the physical panel divided by the display scale — and at 100%… - Screen dimensions are a plausible physical size
screen-plausibleA pure sanity range on screen.width and screen.height: both must be at least 200px and no more than 16384px.
Clearcote is a browser built for fingerprint coherence
It is a Chromium fork, maintained by the same people who wrote this reference. It ships as a compiled browser rather than as a stealth script injected into someone else's — which is a description of how it is built, and is not an argument about how it behaves on this check.
This audit takes no position on how Clearcote scores on Screen & display checks, on this one, or anywhere else. It has no baseline corpus of other people's fingerprints to rank you against and no vendor scoreboard — nearly every check is self-referential, asking one browser the same question through two independent APIs and reporting whether both answers can be true at once. It runs identically on any browser, including ours. Run it on yours and read the result yourself.
See the other checks in Screen & display — the family frameless-covering-is-fullscreen belongs to.