discourse/plugins/chat/assets/stylesheets/common/chat-height-mixin.scss

@mixin chat-height($inset: 0px) {
  // desktop and mobile
  // -1px is for the bottom border of the chat navbar
  $base-height: calc(
    var(--composer-vh, 1vh) * 100 - var(--main-outlet-offset, 0px) - 1px -
      $inset
  );

  height: calc($base-height - var(--composer-height, 0px));

  // mobile with keyboard opened
  html.keyboard-visible & {
    height: calc($base-height);
  }

  // ipad
  html.footer-nav-ipad & {
    height: calc($base-height - var(--composer-height, 0px));
  }

  // PWA/HUB without keyboard
  html.footer-nav-visible:not(.keyboard-visible) & {
    height: calc(
      $base-height - var(--composer-height, 0px) - var(--footer-nav-height, 0px)
    );
  }
}