discourse/plugins/chat/assets/stylesheets/common/chat-message-images.scss

$max_image_height: 150px;

.chat-message {
  // append selectors to set images to a
  // max height of $max_image_height
  .onebox img:not(.ytp-thumbnail-image, .onebox-avatar-inline),
  img.onebox,
  .chat-uploads img,
  p img,
  aside.onebox .onebox-body .aspect-image-full-size,
  aside.onebox .onebox-body .aspect-image-full-size img,
  .chat-message-text p img:not(.emoji) {
    object-fit: contain;
    max-height: $max_image_height;
    max-width: 100%;
    width: unset;
    overflow: hidden;
  }

  .onebox {
    container-type: inline-size;

    .thumbnail {
      &.onebox-avatar {
        max-height: 100px;
        width: 20%;
        max-width: 60px;
        margin-right: 0.5rem;
      }
    }

    @container (width < 400px) {
      .onebox-body {
        &:not(:has(.thumbnail.onebox-avatar)) {
          display: flex;
          flex-direction: column;
        }

        h3 {
          margin-block: 0.75rem 0;
        }
        p {
          margin-top: 0.5rem;
        }

        .thumbnail {
          max-width: 100% !important;
          margin: 0;

          &.onebox-avatar {
            max-width: 20%;
            margin-right: 0.5rem;
          }
        }
      }
    }
  }

  .chat-message-collapser
    .chat-message-collapser-header
    + div
    .chat-message-collapser-lazy-video {
    object-fit: contain;
    max-height: $max_image_height;
    max-width: calc(#{$max_image_height} / 9 * 16);
  }

  // Prevent overflow of old lazy-yt images
  // TODO: remove in December 2023
  .lazyYT.lazyYT-container {
    border: none;
    a {
      display: flex;
    }
    .ytp-thumbnail-image {
      object-fit: contain;
      height: $max_image_height;
      width: calc(#{$max_image_height} / 9 * 16);
      pointer-events: none;
    }
  }
}