user2250471 user2250471 - 1 month ago 5
CSS Question

CSS - "position: fixed" acting like "absolute" in Firefox

I've been building a website in Safari, and I've just tested it in Firefox and my fixed navigation elements are behaving as if they're position is absolute.

#navigation {
display: block;
width: 100%;
height: 50px;
position: fixed;
left: 0px;
bottom: 0px;
text-align: center;
z-index: 99000;
}


This is the CSS I have for the primary navigation wrapper (it's a bottom nav.). In Webkit, it works perfectly: that is, it sticks to the bottom of the window regardless. In firefox, it positions itself at the end of the tags, so, for example, on a long page, I'd have to scroll down just to see it. It is acting as if it's absolute.

I also have a sidebar navigation.

.slidebar {
display: block;
position: fixed;
left: -1px;
top: -1px;
width: 1px;
height: 100%;
overflow: hidden;
-webkit-transition: all 300ms ease;
-moz-transition: all 300ms ease;
-o-transition: all 300ms ease;
-ms-transition: all 300ms ease;
transition: all 300ms ease;
z-index: 99998;
}


This sidebar is also acting as if it's absolute - that is, it is positioning itself off the screen properly, but it's elongating
<body>
and thus the horizontal scrollbar appears. The
height: 100%;
is also responding to the
<body>
height and not the window height, so, for example, my
<header>
has a top margin of 20px, and the slidebar observes that margin too (the body has 0 margin). Likewise, instead of the
height: 100%;
ending at the bottom of the window, it ends at the bottom of the
<body>
, factoring in the footer's bottom margin.

I cannot understand for the life of me why this is happening. Element inspection shows all the properties are loading fine, and in Chrome and Safari it works. It worked initially, and it worked the last time I even edited either navigation, but it has since stopped working since I built other, irrelevant, parts of the site.

http://www.upprise.com/demo.php - click the Envelope icon to see the sidebar

Answer

Through the process of elimination I was able to determine that having the following in my body was causing all the problems with fixed divs in Firefox:

-o-backface-visibility: hidden;
-moz-backface-visibility: hidden;
-webkit-backface-visibility: hidden;
backface-visibility: hidden;

I had originally added this code to prevent flickering in certain CSS transitions throughout the site, but I guess I'll have to add it to each individual class now.

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