CB Activity provides Notifications and it more or less already displays and functions like that. Example as follows.
CB Activity is template driven so you are free to customize it however you like. Basically add a border and remove the spacing between containers and it'll look nearly the same, which should be doable entirely with CSS. Example as follows.
Code:
.cb_template .notificationsStreamItems {
border: 1px solid #dee2e6;
}
.cb_template .notificationsStreamItems > .notificationContainer {
border-color: transparent !important;
margin: 0 !important;
}
.cb_template .notificationsStreamItems > .notificationContainer + .notificationContainer {
border-top: 1px solid #dee2e6 !important;
}
The above would display as follows.
You can find the default template files below.
/components/com_comprofiler/plugin/user/plug_cbactivity/templates/default
However I do not recommend doing so right now as CB Activity 5.0.0 is almost complete and it will be including a full rewrite of the template files to be easier to modify and allow for per-stream templates instead of single global template.