div shifts to the right when drop down menu appears - html

Question

So I have an image inside a div that is centered in an outer-wrapper. I have a horizontal menu at the top of the wrapper that displays 5 sub divs inline. I have added css for a drop down menu that appears when you hover over the first of the 5 inline sub divs. When the drop down menu appears, it causes the image to shift to the right and I can't for the life of me figure out how to correct it. As you can see I have played around with z-index but i'm not sure if I understand what is happening or not happening with z-index and how it is used correctly.
HTML:
<div class="wrapper">
<div id="topmenu">
<div id="home">Home
<ul>
<li>item 1</li>
<li>item 2</li>
<li>item 3</li>
</ul>
</div>
</div>
<div id="logo">
<img src="image.jpeg" />
</div>
</div>
CSS
.wrapper{
position:relative;
width:960px;
height:905px;
margin-top:5px;
margin-left:auto;
margin-right:auto;
/*text-align:left;
border:2px solid red;*/
background-color:#FFFFFF;
}
#topmenu{
position:relative;
border-bottom:2px solid #164207;
height:30px;
background-color:#ffffff;
z-index:3;
}
#logo{
position:relative;
border-bottom:2px solid #164207;
}
#logo img{
position:relative;
height:350px;
width:500px;
z-index:1;
}
#home{
display:inline-block;
float:left;
margin-top:5px;
margin-left:15px;
width:169px;
color:#164207;
font-family:serif;
font-weight:bold;
font-size:20px;
text-align:center;
border-right:2px solid #164207;
}
#home:hover .sub-menu {display:inline-block;}
.sub-menu {
overflow:hidden;
display: none;
width: 169px;
background-color: #164207;
color:#FFFFFF;
margin-top: 5px;
border: 1px solid #fff;
-webkit-box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
-moz-box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
}
.sub-menu li {
position:relative;
list-style-type: none;
display: block;
border-bottom: 1px solid #FFFFFF /*#eaeaea*/;
font-family:serif;
font-size: 12px;
height: 15px;
padding: 8px 0;
}

Answer 1

You need to add position: absolute; to .sub-menu to create a stacking context.
jsFiddle
#home{
display:block;
float:left;
margin-top:5px;
width:184px;
color:#164207;
font-family:serif;
font-weight:bold;
font-size:20px;
text-align:center;
border-right:2px solid #164207;
}
#home:hover .sub-menu {display:block;}
#course:hover .sub-menu{display:block;}
#leagues:hover .sub-menu{display:block;}
#events:hover .sub-menu{display:block;}
#about:hover .sub-menu{display:block;}
.sub-menu {
overflow:hidden;
display: none;
width: 182px;
background-color: #164207;
color:#FFFFFF;
/*padding: 10px 10px;
margin-left: 0px;*/
margin-top: 5px;
border: 1px solid #fff;
-webkit-box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
-moz-box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
box-shadow: 0px 13px 25px rgba(0,0,0, 0.2);
position:absolute; /**************************** Important Bit*/
top:24px;
}
To re-adjust alignment- Remove margin-left:15px; from #home, #course, #leagues, #events, and #about and adjust widths on all of them. Then adjust the width of .sub-menu. See updated jsFiddle above for details and working model.
Example with navigation set up in <ul> unordered list </ul>. - Don't Need all the id's and resulting redundant CSS.