Mica

Mica is the background Windows 11 gives to long lived windows: Settings, File Explorer, the apps you leave open all day. It is opaque, so nothing behind the window is ever legible through it, and it is personalised, because its neutral is mixed with a colour taken from the desktop wallpaper underneath. Move the window across a wallpaper and the tint shifts with it. Leave the window and it drops the tint entirely, falling back to a plain neutral fill, which is how a screen full of windows says which one is listening to your keyboard. Mica Alt is the same material with the wallpaper pushed harder, meant for apps whose title bar carries tabs and needs to separate from the commanding area below.

The Fluent material family is worth keeping straight, because the three names are routinely used as if they meant one thing. Mica samples the wallpaper once and paints an opaque fill from it, so what you see through a Mica window is nothing at all. Acrylic is translucent and blurs live, recomputing as content moves behind it, which is why Microsoft reserves it for transient surfaces such as flyouts and menus rather than for the app backdrop. Glassmorphism is the genre both of those belong to as a look, the general translucent blurred panel that arrived in interface fashion around 2020. Mica is the odd one out of the three, since it is the only member that is not see through.

The sampling rule is the whole engineering argument. A live blur has to be recomposited whenever anything under it moves, and an app backdrop is the largest surface in the window, so the cost lands on every frame for the whole life of the app. Mica captures the wallpaper once, which is why Microsoft describes it as designed for performance and why it is the material recommended for backdrops while acrylic is not. Mica also knows when to stop: it collapses to a solid fill when the user turns transparency off, when battery saver is on, on low end hardware, in High Contrast mode, and whenever the window is inactive. Every one of those is the material admitting that personalisation is the first thing worth spending.

Borrowing the idea outside Windows is harder than it looks, because the web has no access to the desktop wallpaper, and an opaque fill tinted by an image you cannot read is a strange thing to fake. What does travel is the principle: derive the app’s base surface from something the user already chose, keep it opaque so text contrast is fixed rather than dependent on scroll position, and let focus be the thing the colour reports. That last part is the piece most often skipped, and it is the one carrying real information.