Screen resolution

A page never sees your monitor. It sees four numbers the browser reports about it, and a fifth number about the window you opened.

• The size

  Width and height in layout points. This is the reading people mean by resolution, and it is the coarsest of the five.

• The usable area

  The same size minus the bar the desktop keeps for itself. The difference states which system you are on.

• The pixel ratio

  How many device pixels go into one layout point. Multiplied by the size it names the physical panel.

• The colour depth

  Bits per pixel. Almost every machine answers the same here, which makes any other answer loud.

• The window

  The inner size of the page area, which changes as you resize and is more specific than the screen behind it.

These values are read in your browser on this page. Nothing leaves it and no request goes out with the reading.

An edited size is easy. Keeping it from arguing with the four readings around it is the part that decides whether the profile holds.

• A window wider than its screen

  No device can open a window larger than the display it claims to have, and the comparison is one line of page code.

• A size and ratio that multiply to nothing

  A claimed panel that nobody manufactures is as readable as a wrong size, and on Windows the two numbers multiply exactly.

• Portrait sizes with landscape orientation

  A phone taller than it is wide reporting a sideways orientation is a device arguing with itself.

• A usable area above the screen

  The area left after the system bar cannot be larger than the screen it was taken from.

All five readings arrive as one seed on the launch line, before the first line of page code runs. The size comes from panels that machines of that platform really report.

The usable height is the screen minus the bar that system draws, and on a phone there is no bar, so the two are equal. The pixel ratio is drawn from those possible for the size that fell to the profile.

1. 1

   The pair is kept together

   A hand-picked ratio that does not fit the chosen size is refused when you save, and the field that was refused is named.

2. 2

   A mobile profile gets the window too

   The window is brought to the size of the device, and every new tab is held until that size is set.

3. 3

   The set stays with the profile

   The same profile reads the same screen on every launch, because the numbers are derived rather than stored.

Three measurements rather than three claims. The first comes from the acceptance run of the build shipping today, on a Windows profile.

• The size matched the seed

  Measured on a build host whose own display is nothing like the one the profile claims.

  1280x720

• A phone window fits its own device

  A desktop engine keeps windows above five hundred points wide, so the size is set through the debugging protocol instead.

  360x800

• No screen decides the rest

  Across four hundred profiles every screen size occurs with more than one graphics card, so the size alone links nothing.

  many cards per size

The panel above shows what this browser reports. The free check on this site adds the pixel ratio and the colour depth on the same row.

Inside the app the self-test runs in a profile against the engine that profile launched, and the mobile lines also compare the window to the screen.