twbs--bootstrap/scss/mixins/_grid.scss

/// Grid system
//
// Generate semantic grid columns with these mixins.

@mixin make-row($gutter: $grid-gutter-width) {
  display: flex;
  flex-wrap: wrap;
  margin-right: -$gutter / 2;
  margin-left: -$gutter / 2;
}

@mixin make-col-ready($gutter: $grid-gutter-width) {
  // Prevent columns from becoming too narrow when at smaller grid tiers by
  // always setting `width: 100%;`. This works because we set the width
  // later on to override this initial width.
  flex-shrink: 0;
  width: 100%;
  max-width: 100%; // Prevent `.col-auto`, `.col` (& responsive variants) from breaking out the grid
  padding-right: $gutter / 2;
  padding-left: $gutter / 2;
}

@mixin make-col($size, $columns: $grid-columns) {
  flex: 0 0 auto;
  width: percentage($size / $columns);
}

@mixin make-col-auto() {
  flex: 0 0 auto;
  width: auto;
}

@mixin make-col-offset($size, $columns: $grid-columns) {
  $num: $size / $columns;
  margin-left: if($num == 0, 0, percentage($num));
}

// Row columns
//
// Specify on a parent element(e.g., .row) to force immediate children into NN
// numberof columns. Supports wrapping to new lines, but does not do a Masonry
// style grid.
@mixin row-cols($count) {
  & > * {
    flex: 0 0 auto;
    width: 100% / $count;
  }
}

// Framework grid generation
//
// Used only by Bootstrap to generate the correct number of grid classes given
// any value of `$grid-columns`.

@mixin make-grid-columns($columns: $grid-columns, $gutter: $grid-gutter-width, $breakpoints: $grid-breakpoints) {
  @each $breakpoint in map-keys($breakpoints) {
    $infix: breakpoint-infix($breakpoint, $breakpoints);

    @include media-breakpoint-up($breakpoint, $breakpoints) {
      // Provide basic `.col-{bp}` classes for equal-width flexbox columns
      .col#{$infix} {
        flex: 1 0 0%; // Flexbugs #4: https://github.com/philipwalton/flexbugs#flexbug-4
        min-width: 0; // See https://github.com/twbs/bootstrap/issues/25410
      }

      @for $i from 1 through $grid-row-columns {
        .row-cols#{$infix}-#{$i} {
          @include row-cols($i);
        }
      }

      .col#{$infix}-auto {
        @include make-col-auto();
      }

      @for $i from 1 through $columns {
        .col#{$infix}-#{$i} {
          @include make-col($i, $columns);
        }
      }

      // `$columns - 1` because offsetting by the width of an entire row isn't possible
      @for $i from 0 through ($columns - 1) {
        @if not ($infix == "" and $i == 0) { // Avoid emitting useless .offset-0
          .offset#{$infix}-#{$i} {
            @include make-col-offset($i, $columns);
          }
        }
      }
    }
  }
}