wagtail/client/scss/components/forms/_field-row.scss

.w-field-row {
  @include max-form-width();
  gap: theme('spacing.5');
  // For mobile viewports, we attempt to display all items in the row side by side even if not at the desired size.
  display: flex;
  flex-wrap: wrap;

  @include media-breakpoint-up(md) {
    display: grid;
    grid-auto-flow: column;
    // All columns will be the same size.
    grid-auto-columns: minmax(min-content, 1fr);
  }

  .w-field__wrapper + &,
  + .w-field__wrapper {
    margin-top: theme('spacing.5');
  }

  > .w-field__wrapper {
    margin-top: 0;
  }

  // Support explicit sizing of child columns.
  // This isn’t equivalent to 12-column grid – instead, there will be as many columns as calculated from usage of those classes.
  // For example, a `<div class="col6"/><div class="col3"/>` will generate a 9-column grid, with one item taking two thirds of the space.
  // Rather than a 12-column grid with one item taking half and the other a quarter.
  @for $i from 1 through 12 {
    > .col#{$i} {
      display: block;
      float: none;
      padding: 0;
      width: auto;
      grid-column: span #{$i};
    }
  }
}

// Each column will be as wide as its widest member.
.w-field-row--max-content {
  @include media-breakpoint-up(md) {
    grid-auto-columns: minmax(min-content, max-content);
  }
}