Angular ui-router having sticky views with nested views - angularjs

Question

I have a Shell state with two nested states Home and About. I wish to be able to open a Modal state from either Home or About without Home or About being refreshed or removed from the screen, as is the case now. How can I amend to get this working?
http://plnkr.co/edit/EWDN3mmd4Nw0ATITrep4?p=preview
<div class="container">
<div ui-view></div>
</div>
<script type="text/ng-template" id="/shell.html">
<ul class="nav nav-pills">
<li>Home</li>
<li>About</li>
<li><a class="btn btn-default" ui-sref="shell.modal" data-toggle="modal" data-target="#modal">Open Modal</a></li>
</ul>
<div ui-view="main"></div>
<div class="modal fade" id="modal">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<a class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</a>
</div>
<div class="modal-body">
<div ui-view="modal"></div>
</div>
</div>
</div>
</div>
</script>
<script type="text/ng-template" id="/home.html">
<h1>Home</h1>
<p>{{now}}</p>
</script>
<script type="text/ng-template" id="/about.html">
<h1>About</h1>
<p>{{now}}</p>
</script>
<script type="text/ng-template" id="/modal.html">
<h1>Modal content</h1>
<p>{{now}}</p>
</script>
<script>
var modalApp = angular.module('modalApp', ['ui.router']);
modalApp.config(function ($stateProvider, $urlRouterProvider) {
$urlRouterProvider.otherwise('/home');
$stateProvider
.state('shell', {
templateUrl: '/shell.html',
controller: function($scope){
$scope.now = Date.now();
}
})
.state('shell.home', {
url: '/home',
views: {
'main#shell': {
templateUrl: '/home.html',
controller: function($scope){
$scope.now = Date.now();
}
}
}
})
.state('shell.about', {
url: '/about',
views: {
'main#shell': {
templateUrl: '/about.html',
controller: function($scope){
$scope.now = Date.now();
}
}
}
})
.state('shell.modal', {
views: {
"modal#shell": {
templateUrl: '/modal.html',
controller: function($scope) {
$scope.now = Date.now();
}
}
}
});
});
</script>
(I do not want to use the $modal service from Angular UI as I have similar nested view requirements that are not using modals.)