Polymer: Buttonclick triggers parent container click-event - polymer

Question

I'm trying to develop a collapsible list based on Google Inbox in Polymer.
I've created a list of polymer-elements, where each element is a paper-material with an iron-collapse inside.
The iron-collapse contains data and a couple of paper-buttons.
I need to show/hide the <iron-collapse> element, when clicking the parent paper-material.
This works really well, but unfortunately that event also fires after clicking a paper-button e.g. Send-Button inside the iron-collapse.
I tried adding event.stopPropagation() to the child's button handler, but the iron-collapse still collapses, when clicking the button.
Any ideas?
Code for parent container:
<paper-material id="papercontainer" elevation="1" class="container padded" animated style="border-radius:4px;">
<div class="container">
<div class="container flex-start-justified">
<div class="flexchild">
<h4 class="smaller-margin">Bill Gates</h4>
<p class="smaller-margin">bill#gates.com</p>
<p class="paper-font-caption smaller-margin">Date received: 01/01/2015</p>
</div>
<p>
<paper-button raised on-click="send">Send</paper-button>
</p>
</div>
<!--Code for iron-collapse (child): -->
<div class="container flex-horizontal">
<iron-collapse id="collapse" class="flexchild">
<div class="flexchild collapse-content" style="margin-top:10px;">
<paper-button>Edit Mail</paper-button>
</div>
</iron-collapse>
</div>
</div>
</paper-material>
Javascript:
Polymer({
is: 'zapytania-result-element2',
toggle: function() {
this.$.collapse.toggle();
},
listeners: {
'tap': 'regularTap'
},
regularTap: function(e) {
console.log('toggle iron-collapse');
if(this.$.collapse.opened) {
this.$.collapse.hide();
this.$.papercontainer.elevation = 1;
} else {
this.$.collapse.show();
this.$.papercontainer.elevation = 5;
}
},
send: function() {
sendMail();
}
}

Answer 1

I have followed the Events Tutorial from
https://www.polymer-project.org/1.0/docs/devguide/events.html
Per my understanding 'tap' makes the whole element listen for the tap event.
After removing that handler and assigning my own on-click event handler to the parent paper-material container and calling e.stopPropagation() at the end of my child's button event it works.