Ok, I've found it and created a very simple testcase: shortly, I think the problem is that Calibre ebook viewer calculates the width of an element summing the widths of all the pages that element spans:
PHP Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr">
<head>
<title>Test</title>
<style type="text/css">
#container
{
width: 500px;
overflow: hidden;
}
#content
{
position: relative;
}
</style>
</head>
<body>
<div id="container">
<div id="content">
1<br />
2<br />
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</div>
</div>
</body>
</html>
To be concise, the problem rises when a #content div is the child of a #container div; #container has a not-auto width and hides overflows, while #content is relative positioned and its contents span more than one ebook page.
Notice that if you set #container width to 1000px you can see the second page but not the third, that's why I think the viewer wrongs to calculate the element width. It's not clear why this happens only when #content is relative positioned.