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[CSS21] Contradiction in the spec with auto-offset fixed-position elements

From: Boris Zbarsky <bzbarsky@MIT.EDU>
Date: Sat, 06 Nov 2004 21:24:34 -0500
Message-ID: <418D8762.20800@mit.edu>
To: www-style@w3.org

This came up recently in a bug filed on Mozilla... [1]  Consider the 
following situation:

<root>
<node style="position: fixed">Fixed</node>
Lots
of
content
on
many
lines.
</root>

When laying this out, per section 10.6.4 [2] the UA must compute the 
hypothetical box of the <node /> element, then figure out the offset of 
that box from the top of the viewport, and use that offset as the 
computed value for "top".

The problem is that scrolling should not affect the position of the 
<node /> element (since it is fixed-positioned), while scrolling affects 
the hypothetical box (for example, scrolling to the bottom of the 
document could put the entire hypothetical box above the top of the 
viewport).

This means that if the document is scrolled and then the viewport size 
is changed (which necessitates a recalculation of the offsets of 
<node>), the fixed-positioned element will actually move.  In UAs that 
recalculate the offsets more eagerly, it could even move when scrolling.

This does not seem at all desirable, in my opinion.  Would it not make 
more sense to specify that for fixed positioning auto top and left are 
replaced by 0 before doing the other calculations?  There is no issue of 
compatibility with IE/Windows here, unlike absolute positioning, since 
IE/Windows doesn't support fixed positioning...  I really don't see any 
way to use this "hypothetical box" concept for fixed-positioned boxes 
without running contradictions.

-Boris

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=268111
[2] http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-height
Received on Sunday, 7 November 2004 02:24:49 GMT

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