wagtail/client/scss/components/forms/_form-width.scss

.w-form-width {
  @include max-form-width();
}

@include media-breakpoint-up(md) {
  .minimap-open {
    .w-form-width {
      max-width: theme('width.[4/5]');
    }
  }

  .side-panel-open {
    .w-form-width {
      // Account for dynamic width of the side panel when open.
      max-width: max(
        theme('width.[1/3]'),
        calc(100% - var(--side-panel-width))
      );
    }
  }

  .side-panel-open.minimap-open {
    .w-form-width {
      // Account for additional space taken up by the minimap.
      max-width: max(
        theme('width.[2/5]'),
        calc(100% - var(--side-panel-width) - 15rem)
      );
    }
  }
}

.fields {
  // Apply the desired form width for legacy `fields` container.
  max-width: $max-form-width;

  // Remove any spacing in legacy fields markup.
  > li {
    padding: 0;
  }
}