When 1 px border is added to div, Div size increases, Don't want to do that

This is also helpful in this scenario. it allows you to set borders without changing div width

textarea { 
  -webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
  -moz-box-sizing: border-box;    /* Firefox, other Gecko */
  box-sizing: border-box;         /* Opera/IE 8+ */
}

Taken from http://css-tricks.com/box-sizing/

The border css property will increase all elements "outer" size, excepts tds in tables. You can get a visual idea of how this works in Firebug (discontinued), under the html->layout tab.

Just as an example, a div with a width and height of 10px and a border of 1px, will have an outer width and height of 12px.

For your case, to make it appear like the border is on the "inside" of the div, in your selected CSS class, you can reduce the width and height of the element by double your border size, or you can do the same for the elements padding.

Eg:

div.navitem
{
    width: 15px;
    height: 15px;
    /* padding: 5px; */
}

div.navitem .selected
{
    border: 1px solid;
    width: 13px;
    height: 13px;
    /* padding: 4px */
}

set a border on it before you click to be the same color as the background.

Then when you click just change the background color and the width will not change.