Extra space in running header caused by \label or \index

[FrankMittelbach]

Brief outline of the bug

This bug is older than 35 years ...

Minimal example showing the bug

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{book}

\makeindex
\pagestyle{headings}
\begin{document}
\showoutput

\tableofcontents

\chapter{X \index{blub} X  X}
\section{Y \label{bla} Y  Y}

Shows 2 spaces in running header, but one (correct) in TOC
\newpage x \newpage x \newpage
\end{document}

Reason: LaTeX used \@gobble to silence \index and friends, which is fine if written to a file (hence TOC is ok) but not if the data ends up in a mark. Then you get spaces from both sides of the gobbled command.

Log file (required) and possibly PDF file

From the log:

.....\OT1/cmr/m/sl/10 1
.....\OT1/cmr/m/sl/10 .
.....\OT1/cmr/m/sl/10 1
.....\OT1/cmr/m/sl/10 .
.....\glue 4.44444 plus 4.99997 minus 0.37036
.....\glue 3.33333 plus 1.66666 minus 1.11111
.....\OT1/cmr/m/sl/10 Y
.....\glue 3.33333 plus 1.66498 minus 1.11221
.....\glue 3.33333 plus 1.66498 minus 1.11221
.....\OT1/cmr/m/sl/10 Y
.....\glue 3.33333 plus 1.66498 minus 1.11221
.....\OT1/cmr/m/sl/10 Y