How to prevent inline-block divs from wrapping?

Add white-space: nowrap; to your .layout style declaration.

This will do exactly what you need: preventing the divs from wrapping.

Watch the

jsFiddle demo

or run the following snippet full screen and resize it:

.layout {
       white-space : nowrap; /* this does the trick */
          overflow : hidden; /* this prevents the grey divs from overflowing */
    vertical-align : top;
     border-radius : 15px;
           display : inline-block;
}

.layoutbacking {
    border-radius : 15px;
       background : #CCCCCC;
          padding : 5px;
           margin : 5px;
}
<div class="layout" style="background: #222222; width: 100%">
    <div>
        <div class="layout layoutbacking">
            <table>
                <tr>
                    <th>header 1</th>
                    <th>header 2</th>
                    <th>header 3</th>
                    <th>header 4</th>
                </tr>
                <tr>
                    <td>data 1</td>
                    <td>data 2</td>
                    <td>data 3</td>
                    <td>data 4</td>
                </tr>
                <tr>
                    <td>data 1</td>
                    <td>data 2</td>
                    <td>data 3</td>
                    <td>data 4</td>
                </tr>
                <tr>
                    <td>data 1</td>
                    <td>data 2</td>
                    <td>data 3</td>
                    <td>data 4</td>
                </tr>
            </table>
        </div>
        <div class="layout">
            <div class="layout layoutbacking">
                <table>
                    <tr>
                        <th>header 1</th>
                        <th>header 2</th>
                        <th>header 3</th>
                        <th>header 4</th>
                    </tr>
                    <tr>
                        <td>data 1</td>
                        <td>data 2</td>
                        <td>data 3</td>
                        <td>data 4</td>
                    </tr>
                </table>
            </div>
            <br />
            <div class="layout layoutbacking">
                <table>
                    <tr>
                        <th>header 1</th>
                        <th>header 2</th>
                        <th>header 3</th>
                        <th>header 4</th>
                    </tr>
                    <tr>
                        <td>data 1</td>
                        <td>data 2</td>
                        <td>data 3</td>
                        <td>data 4</td>
                    </tr>
                </table>
            </div>
        </div>
    </div>
    <div>
        <div class="layout layoutbacking">
            <table>
                <tr>
                    <th>header 1</th>
                    <th>header 2</th>
                    <th>header 3</th>
                    <th>header 4</th>
                </tr>
                <tr>
                    <td>data 1</td>
                    <td>data 2</td>
                    <td>data 3</td>
                    <td>data 4</td>
                </tr>
            </table>
        </div>
        <div class="layout layoutbacking">
            <table>
                <tr>
                    <th>header 1</th>
                    <th>header 2</th>
                    <th>header 3</th>
                    <th>header 4</th>
                </tr>
                <tr>
                    <td>data 1</td>
                    <td>data 2</td>
                    <td>data 3</td>
                    <td>data 4</td>
                </tr>
            </table>
        </div>