Book HomeCascading Style Sheets: The Definitive GuideSearch this book Thursday 03rd of September 2015 01:42:39 AM

9.4. Fixed Positioning

As implied in the previous section, fixed positioning is just like absolute positioning, except the containing block of a fixed element is always the viewport. In this case, the element is totally removed from the document's flow and does not have a position relative to any part of the document.

<P STYLE="height: 10em;">

In this case, then the extra height is treated somewhat like extrapadding, as depicted in Figure 8-3.

Figure 8-3

Figure 8-3. Setting the height property for block-level elements

If, on the other hand, the height isless than that needed to display the content:

<P STYLE="height: 3em;">

This can be exploited in a number of interesting ways. First off, it's possible to create frame-style interfaces using fixed positioning. Consider Figure 9-24, which shows a very common layout scheme.

Figure 9-24

Figure 9-24. Emulating frames with fixed positioning

This could be done using the following styles:

DIV#header {position: fixed; top: 0; bottom: 80%; left: 20%; right: 0;
background: gray;}
DIV#sidebar {position: fixed; top: 0; bottom: 0; left: 0; right: 80%;
background: silver;}

This will fix the header and sidebar to the top and side of the viewport, where they will remain regardless of how the document is scrolled. The drawback here, though, is that the rest of the document will be overlapped by the fixed elements. Therefore, the rest of the content should probably be contained in its own DIV and employ the following:

DIV#main {position: absolute; top: 20%; bottom: 0; left: 20%; right: 0;
overflow: scroll; background: white;}

It would even be possible to create small gaps between the three positioned DIVs by adding some appropriate margins, demonstrated in Figure 9-25:

BODY {background: black; color: silver;}  /* colors for safety's sake */
DIV#header {position: fixed; top: 0; bottom: 80%; left: 20%; right: 0;
background: gray; margin-bottom: 2px; color: yellow;}
DIV#sidebar {position: fixed; top: 0; bottom: 0; left: 0; right: 80%;
background: silver; margin-right: 2px; color: maroon;}
DIV#main {position: absolute; top: 20%; bottom: 0; left: 20%; right: 0;
overflow: scroll; background: white; color: black;}
Figure 9-25

Figure 9-25. Separating the "frames"

Given such a case, a tiled image could be applied to the BODY background. This image would show through the gaps created by the margins, which could certainly be widened if the author saw fit. For that matter, if a background image was of little importance, simple borders could be applied to the DIVs instead of margins.



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TIP

There are serious issues related to document reflow with :hover and :focus. Take, for example:

A:hover {font-size: 200%;}

In theory, a user agent would have to double the size of anchor text as the pointer hovers over it, which could well cause major redisplay issues. An author could cause similar problems by declaring that TEXTAREA elements should change their size when

Here we have a case where the element should be 40% as wide as thecontaining block but can never be less than 15emwide. We've also changed the bottom andheight so that they're automaticallydetermined. This will let the element be as tall as necessary todisplay its content, no matter how narrow it gets (never less than15em, of course!).

We can turn this around to keep elements from getting too wide ortall by using max-width and