Child element shifting parent div - css

Question

I've searched quite a bit looking for an explanation as to why this behavior is occurring.
Essentially I've setup 2 columns, each with a nav bar and content area.
CSS
#mainContainer {
background-color: lightblue;
width: 100%;
height: 300px;
}
#leftContainer, #rightContainer {
border: 1px solid black;
display: inline-block;
background-color: red;
width: 40%;
height: 100%;
}
#leftBar, #rightBar {
background-color: purple;
height: 10%;
}
#leftMain, #rightMain {
background-color: grey;
height: 90%;
}
HTML
<div id="mainContainer">
<div id="leftContainer">
<div id="leftBar"></div>
<div id="leftMain"></div>
</div>
<div id="rightContainer">
<div id="rightBar"></div>
<div id="rightMain"></div>
</div>
</div>
Whenever I add an element to the nav bar in only one column it shifts the entire column down.
http://jsfiddle.net/qn6rs0q2/3/
<div id="mainContainer">
<div id="leftContainer">
<div id="leftBar">
<button>Test</button>
</div>
<div id="leftMain"></div>
</div>
<div id="rightContainer">
<div id="rightBar"></div>
<div id="rightMain"></div>
</div>
</div>
But if I add another element to the other column they line up again.
http://jsfiddle.net/qn6rs0q2/5/
<div id="mainContainer">
<div id="leftContainer">
<div id="leftBar">
<button>Test</button>
</div>
<div id="leftMain"></div>
</div>
<div id="rightContainer">
<div id="rightBar">
<button>Test 2</button>
</div>
<div id="rightMain"></div>
</div>
</div>
To clarify, I'm not looking for a solution to fix this behavior. Rather I'm hoping someone can explain the underlying reason behind why it's behaving as it is. Thanks in advance.

Answer 1

It happens because the default vertical alignment of inline elements is the baseline. If you set the vertical alignment to top (or middle) for both sides, they line up as you want:
#leftContainer, #rightContainer {
border: 1px solid black;
display: inline-block;
background-color: red;
width: 40%;
height: 100%;
vertical-align:top;
}
jsFiddle example