There are other cards affected by this style, like the account settings cards. It also looks like the `- 5.8rem + 0.5rem` is not quite correct for those cards either, just -5rem should enough for those cards.

So I would go with this generic value being `height: calc(100% - 5rem);`, then create a more specific match for the reader card in the message_cards.css under the other .card-message-reader that is just:

```css
.card-message-reader .scrollregion-below-header {
  height: calc(100% - 9.5rem);
}
```