fillMaxWidth

fun Modifier.fillMaxWidth(@FloatRange(from = 0.0, to = 1.0) fraction: Float = 1f) =
    this.then(if (fraction == 1f) FillWholeMaxWidth else FillElement.width(fraction))

Have the content fill (possibly only partially) the Constraints.maxWidth of the incoming measurement constraints, by setting the minimum width and the maximum width to be equal to the maximum width multiplied by fraction. Note that, by default, the fraction is 1, so the modifier will make the content fill the whole available width. If the incoming maximum width is Constraints.Infinity this modifier will have no effect.

Parameters