layoutMediaQueryData function

MediaQueryData layoutMediaQueryData(MediaQueryData media, Size box)

Rebuilds media for a full-screen surface laid out in a box of box size.

Apps that scale their whole tree — ResponsiveScaledBox, a root FittedBox, a Transform.scale — lay every route and overlay entry out on a canvas whose size differs from the screen media describes. Geometry taken from MediaQuery.size then lands in the wrong place on that canvas: a corner window stops short of the corner, a full-screen rect does not fill it. Taking the size from the box, and scaling padding and insets by the same factor, puts everything back in the canvas's own units.

The correction only applies when box has the screen's shape, which is the signature of a uniformly scaled screen. A box of any other shape is a region of the screen (a split pane, a sized slot), whose insets cannot be derived by ratio, so media is returned unchanged — as it is when the sizes already agree, which is every app that does not scale its tree.

Implementation

MediaQueryData layoutMediaQueryData(MediaQueryData media, Size box) {
  final screen = media.size;
  if (!box.isFinite || box.isEmpty || screen.isEmpty || box == screen) {
    return media;
  }
  final scaleX = box.width / screen.width;
  final scaleY = box.height / screen.height;
  if ((scaleX / scaleY - 1).abs() > _uniformScaleTolerance) return media;

  EdgeInsets scaled(EdgeInsets insets) => EdgeInsets.fromLTRB(
        insets.left * scaleX,
        insets.top * scaleY,
        insets.right * scaleX,
        insets.bottom * scaleY,
      );
  return media.copyWith(
    size: box,
    padding: scaled(media.padding),
    viewPadding: scaled(media.viewPadding),
    viewInsets: scaled(media.viewInsets),
    systemGestureInsets: scaled(media.systemGestureInsets),
  );
}