I’m trying to understand CSS positioning and I’m having trouble figuring out why a simple change that apparently should have no effect on the layout is causing a very disruptive change. I’m obviously missing something.
The initial objective was to place an inner div
vertically and horizontally within another div. That was fairly simple:
html, body {
margin:0;
padding:0;
height:100%;
}
div#container {
position:relative;
background:#4444ff;
margin: 0 auto; /* center, not in IE5 */
height:80%;
min-height:80%;
}
div#childDiv {
position:absolute;
background:#ff5555;
/* next we center it vertically and horizontally */
width:900px;
height:600px;
top:50%;
margin-top:-300px;
left:50%;
margin-left:-450px;
}
…and in the HTML page I used:
<body>
<div id=container>
<div id=childDiv>
test
</div>
</div>
</body>
which worked fine.
The curious part is what happens when I change the position attribute of the #childDiv div from absolute to relative.
My understanding is that first is should not affect the #container div at all since I’m changing only the position of the child element, and second that it should not change the layout since it is the only child element, its parent uses relative position and third I have not specified any offsets (tp, left, etc).
Instead, when I make this change, the parent #container is messed up (shows only up to the half of the viewport instead of 80% height as previously), and the position of #childDiv changes accordingly (also upwards, half outside the viewport).
My questions is: why does that happen? What concepts I’m not taking into account and why was the parent div affected by a change in the children’s position setting?
If I remove #childDiv from within #container and place it inside body, then #container is no longer affected by that change so it seems something is propagating up in the DOM, which is odd to me. I’ve seen the same in firefox, opera, IE and chrome.
I have read W3C’s spec on this topic but I haven’t been able to figure this one out so far…
UPDATE: I created examples in JS fiddle to show the problem. You can see the original is here: jsfiddle.net/7Pr9y/1 and the affected one is here: jsfiddle.net/7Pr9y/3
Thank you!
Eduardo
When something is absolutely positioned, it is taken out of normal flow so its size, margins, etc. do not affect the things around it.
When something is relatively positioned, it is placed in normal flow (so its size, margins, etc. do affect the things around it) and layout is initially handled as if it were
position: static, then it is moved according to theleft,right,topandbottomproperties.