@blueprintjs/core/src/common/_flex.scss

// Copyright 2018 Palantir Technologies, Inc. All rights reserved.
// Licensed under the Apache License, Version 2.0.

// this element becomes a flex container in the given direction.
// supply `$margin` to put space between each child.
// supply `$inline: inline` to use `display: flex-inline`.
// customize `flex: 1 1` child selector with $fill.
@mixin pt-flex-container($direction: row, $margin: none, $inline: none, $fill: ".#{$ns}-fill") {
  @if $inline == inline or $inline == true {
    display: inline-flex;
  } @else {
    display: flex;
  }
  flex-direction: $direction;

  > * {
    flex-grow: 0;
    flex-shrink: 0;
  }

  > #{$fill} {
    flex-grow: 1;
    flex-shrink: 1;
  }

  @if $margin != none {
    @include pt-flex-margin($direction, $margin);
  }
}

// applies margin along axis of direction between every direct child, with no margins on either end.
// $direction: row | column
// $margin: margin[-right|-bottom] value
@mixin pt-flex-margin($direction, $margin) {
  $margin-prop: if($direction == row, margin-right, margin-bottom);

  // CSS API support
  &::before,
  > * {
    // space after all children
    #{$margin-prop}: $margin;
  }

  // remove space after last child
  &:empty::before,
  > :last-child {
    #{$margin-prop}: 0;
  }
}