Why do "inline-block" elements in a "nowrap" div overflow?

Question

The following code causes #headline to overflow #wrapper and I do not understand why this is happening.

HTML:

<div id="wrapper">
    <div id="logo">
        <img src="/test.png">
    </div>
    <div id="headline">
        <h1>This headline overflows its wrapping div. # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #</h1>
    </div>
</div>

CSS:

#wrapper {
    background: #fea;
    white-space: nowrap;
    width: 50%;
}

#logo {
    display: inline-block;
    vertical-align: middle;
}

#logo img {
       width: 6em; 
}

#headline {
     display: inline-block;
     vertical-align: middle;
     white-space: normal;
}

Example code: http://jsfiddle.net/XjstZ/21/ or http://tinkerbin.com/XvSAEfrK

Answer 1

As the name suggests, inline-blocks participate in inline layout, which means they act just like inline elements, and text. white-space: nowrap causes text to overflow in an element by preventing it from wrapping; the same thing happens with inline-blocks. It's that simple.

The fact that #headline has white-space: normal has no impact on its own layout. An element's white-space property affects the layout of its contents, not itself, even if the element's own box is inline-level.