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The headers display information such as map title, part title, chapter title, and section title, ending in the page number.

content: string(maptitle) string(parttitle) string(chaptertitle) string(sectiontitle) " | " counter(page);

This might be too much if you have long titles. The solution is to override the default header content.

In your customization CSS, add the following CSS rule:

@page :left {
    @top-left {
        content: string(chaptertitle) " | " counter(page);
    }
}
@page :right{
    @top-right {
        content: string(chaptertitle) " | " counter(page);
    }
}
Important: Some of the CSS default page rules are more important. If you see that the content does not change:
  • Try to also specify the name of the page, to increase the specificity of the rules:
    @page :left, table-of-contents:left, chapter:left{
      ...
    }
    @page :right, table-of-contents:right, chapter:right{
      ...
    }
  • Add an !important classifier just before the semi-colon.
    @top-right {
        content: string(chaptertitle) " | " counter(page) !important;
    }