Based on @Conaw's tweet. Ref:: https://twitter.com/Conaw/status/1362762067648815108

roam-horizontal-group-css

.group {
  display: flex;
  align-items: center;
  flex-direction: row;
}

.group div {
  border: none;
}

.group > .rm-block__self {
  flex-grow: 0.1;
}

.group .rm-block__children .rm-block__controls {
  opacity: 0;
}

.group:hover  .rm-block__children .rm-block__controls {
  opacity: 1;
}

.group > .rm-block__children {
  flex-grow: 1;
  margin: 12px 4px;
  padding: 8px 2px;
  border-top: 1px solid green;
  border-bottom: 1px solid green;
  border-left: 1px solid green;
}