Strange onclick behavior after add style to div

Question

I have html:

<div>
    <div id='icon_zoom_in' class='icon'>+</div>
    <div id='icon_zoom_out' class='icon'>-</div>
</div>  

And I add CSS:

.icon{
    color: white;
    font-size: 100px;
    background-color: black;
    opacity: 0.7;
    -webkit-border-radius: 20px;
    -moz-border-radius: 20px;
    width: 70px;
    height: 70px;
    text-align: center;
    line-height: 50px;
    cursor: pointer;
}  

The result is nice(ignore the font, I installed a chrome extension):

But when I add click event on there "buttons", strange things happen:

var $ = function(id) {
    return document.getElementById(id);
}
$("icon_zoom_in").addEventListener("click", function() {
    console.log("zoom in");
}, false);
$("icon_zoom_out").addEventListener("click", function() {
    console.log("zoom out");
}, false);  

When I click the "+" button, I got zoom out ! I have to click the outter space of it to get zoom in .
jsfiddle: http://jsfiddle.net/wong2/w2dRB/

Answer 1

Super simple: in Chrome the text is overflowing. Actually when you click the plus you are clicking the minus because of this. Use overflow: hidden; and the plus and minus will stick inside the buttons. Here (JSFiddle) you can test the correct behaviour.