wp-includes/blocks/group/editor.css

.wp-block-group .block-editor-block-list__insertion-point{
  left:0;
  right:0;
}

[data-type="core/group"].is-selected .block-list-appender{
  margin-left:0;
  margin-right:0;
}
[data-type="core/group"].is-selected .has-background .block-list-appender{
  margin-bottom:18px;
  margin-top:18px;
}

.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child{
  gap:inherit;
  pointer-events:none;
}
.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child,.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child .block-editor-default-block-appender__content,.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child .block-editor-inserter{
  display:inherit;
  flex:1;
  flex-direction:inherit;
  width:100%;
}
.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child:after{
  border:1px dashed;
  border-radius:2px;
  content:"";
  display:flex;
  flex:1 0 48px;
  min-height:46px;
  pointer-events:none;
}
.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child:after:before{
  background:currentColor;
  bottom:0;
  content:"";
  left:0;
  opacity:.1;
  pointer-events:none;
  position:absolute;
  right:0;
  top:0;
}
.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child .block-editor-button-block-appender,.wp-block-group.is-layout-flex.block-editor-block-list__block>.block-list-appender:only-child .block-editor-inserter{
  pointer-events:all;
}