Viel zu lange Titel im Header mit scrlayer-scrpage

Schon

\documentclass{book}
\usepackage{scrlayer-scrpage}
\setlength{\headheight}{2\baselineskip}
\begin{document}
\chapter{This is very very long and there are no nicer wombats in here,
which is really really
sad}
\section{This is very very long and there are no capybaras in
here, which is really really sad}
\null\clearpage\null\clearpage\null\clearpage\null\clearpage
\end{document}

funktioniert nicht.

Die Verwendung des linksbündigen, zentrierten oder rechtsbündigen Elements in Kopf oder Fuß bedeutet nicht, dass keine Überdeckung mit anderen Elementen möglich ist. Man muss ggf. selbst dafür sorgen, dass die Elemente sich nicht in die Quere kommen, indem man den Inhalt entsprechend in der Breite beschränkt. Dafür gibt es verschiedene Möglichkeiten. Beispielsweise:

• Ganz klassisch mit einer \parbox:
  

  \documentclass{book}
  \usepackage{scrlayer-scrpage}
  \setlength{\headheight}{2\baselineskip}
  \ihead{\parbox[b]{\dimexpr\textwidth-2em}{\ifodd\value{page}\else\raggedleft\fi\headmark}}
   
  \begin{document}
  \chapter{This is very very long and there are no nicer wombats in here,
  which is really really
  sad}
  \section{This is very very long and there are no capybaras in
  here, which is really really sad}
  \null\clearpage\null\clearpage\null\clearpage\null\clearpage
  \end{document}

• Einfach durch Verminderung der horizontalen Breite:
  

  \documentclass{book}
  \usepackage{scrlayer-scrpage}
  \setlength{\headheight}{2\baselineskip}
  \ihead{\addtolength{\hsize}{-2em}\headmark}
   
  \begin{document}
  \chapter{This is very very long and there are no nicer wombats in here,
  which is really really
  sad}
  \section{This is very very long and there are no capybaras in
  here, which is really really sad}
  \null\clearpage\null\clearpage\null\clearpage\null\clearpage
  \end{document}

• Durch Verkleinern der Kopfbreite und setzen der Seitenzahl in den Kopfrand:
  

  \documentclass{book}
  \usepackage{scrlayer-scrpage}
  \setlength{\headheight}{2\baselineskip}
  \KOMAoptions{headwidth=\dimexpr \textwidth-2em:0pt}
  \lehead*{\makebox[0pt][r]{\makebox[2em][l]{\usekomafont{pagenumber}{\thepage}}}}
  \rohead*{\makebox[0pt][l]{\makebox[2em][r]{\usekomafont{pagenumber}{\thepage}}}}
  \begin{document}
  \chapter{This is very very long and there are no nicer wombats in here,
  which is really really
  sad}
  \section{This is very very long and there are no capybaras in
  here, which is really really sad}
  \null\clearpage\null\clearpage\null\clearpage\null\clearpage
  \end{document}

• Verlagerung der Seitenzahl in den Fuß:
  

  \documentclass{book}
  \usepackage[pagestyleset=KOMA-Script,markcase=upper]{scrlayer-scrpage}
  \setlength{\headheight}{2\baselineskip}
  \begin{document}
  \chapter{This is very very long and there are no nicer wombats in here,
  which is really really
  sad}
  \section{This is very very long and there are no capybaras in
  here, which is really really sad}
  \null\clearpage\null\clearpage\null\clearpage\null\clearpage
  \end{document}

Weitere Möglichkeiten sind denkbar, beispielsweise könnte man die Seitenzahl in einen eigenen Layer verschieben und Position und Breite der Layer scrheadings.head.odd und scrheadings.head.even modifizieren.