Fix second part of #5386.

A \clearpage command issued right before \end{CJK} is recommended by the
package author to prevent any un-processed CJK chars outside the
\begin{CJK} and \end{CJK} scope. Otherwise, TOC, header, footer,
and may contain CJK chars but get processed outside the CJK environment scope.

Tha new dedicated export test fails without the fix.

autotests/export/latex/CJK/zh_CN-toc.lyx

@@ -0,0 +1,175 @@
+#LyX 2.4 created this file. For more info see https://www.lyx.org/
+\lyxformat 566
+\begin_document
+\begin_header
+\save_transient_properties true
+\origin unavailable
+\textclass report
+\begin_preamble
+
+\end_preamble
+\use_default_options false
+\maintain_unincluded_children false
+\language chinese-traditional
+\language_package default
+\inputencoding utf8-cjk
+\fontencoding auto
+\font_roman "lmodern" "Noto Sans CJK SC"
+\font_sans "default" "Noto Sans CJK SC Light"
+\font_typewriter "default" "Noto Sans Mono CJK SC"
+\font_math "auto" "auto"
+\font_default_family default
+\use_non_tex_fonts false
+\font_sc false
+\font_osf false
+\font_sf_scale 100 100
+\font_tt_scale 100 100
+\font_cjk bsmi
+\use_microtype false
+\use_dash_ligatures false
+\graphics default
+\default_output_format pdf2
+\output_sync 0
+\bibtex_command default
+\index_command default
+\float_placement class
+\float_alignment class
+\paperfontsize 12
+\spacing single
+\use_hyperref true
+\pdf_bookmarks true
+\pdf_bookmarksnumbered false
+\pdf_bookmarksopen true
+\pdf_bookmarksopenlevel 1
+\pdf_breaklinks false
+\pdf_pdfborder false
+\pdf_colorlinks true
+\pdf_backref false
+\pdf_pdfusetitle true
+\papersize a4paper
+\use_geometry false
+\use_package amsmath 1
+\use_package amssymb 1
+\use_package cancel 1
+\use_package esint 1
+\use_package mathdots 0
+\use_package mathtools 1
+\use_package mhchem 1
+\use_package stackrel 1
+\use_package stmaryrd 1
+\use_package undertilde 1
+\cite_engine basic
+\cite_engine_type default
+\biblio_style plain
+\use_bibtopic false
+\use_indices false
+\paperorientation portrait
+\suppress_date false
+\justification true
+\use_refstyle 0
+\use_minted 0
+\index 索引
+\shortcut idx
+\color #008000
+\end_index
+\secnumdepth 3
+\tocdepth 3
+\paragraph_separation skip
+\defskip 2ex
+\is_math_indent 0
+\math_numbering_side default
+\quotes_style english
+\dynamic_quotes 0
+\papercolumns 1
+\papersides 1
+\paperpagestyle default
+\tracking_changes false
+\output_changes false
+\html_math_output 0
+\html_css_as_file 0
+\html_be_strict false
+\end_header
+
+\begin_body
+
+\begin_layout Standard
+\begin_inset CommandInset toc
+LatexCommand tableofcontents
+
+\end_inset
+
+
+\end_layout
+
+\begin_layout Chapter
+\SpecialChar LyX
+
+\end_layout
+
+\begin_layout Standard
+This is a Chinese test document for \SpecialChar LyX
+ with the CJK package.
+\end_layout
+
+\begin_layout Standard
+The 
+\emph on
+Table of Contents 
+\emph default
+heading appears as 
+\begin_inset Quotes eld
+\end_inset
+
+Contents
+\begin_inset Quotes erd
+\end_inset
+
+ in the PDF.
+\end_layout
+
+\begin_layout Standard
+Without a 
+\backslash
+clearpage before closing the 
+\begin_inset Quotes eld
+\end_inset
+
+CJK
+\begin_inset Quotes erd
+\end_inset
+
+ environment, Chinese characters in a section title lead to errors like
+\end_layout
+
+\begin_layout LyX-Code
+! Package inputenc Error: Unicode char 中 (U+4E2D) 
+\end_layout
+
+\begin_layout LyX-Code
+(inputenc) not set up for use with LaTeX.
+\end_layout
+
+\begin_layout Standard
+(cf.
+ https://www.lyx.org/trac/ticket/5386).
+\end_layout
+
+\begin_layout Section
+Section in English
+\end_layout
+
+\begin_layout Standard
+This section head is in English.
+\end_layout
+
+\begin_layout Section
+Secition in Chinese 中文名
+\end_layout
+
+\begin_layout Standard
+This section head is in Chinese.
+ 這節的標題是中文。
+\end_layout
+
+\end_body
+\end_document

src/output_latex.cpp

@@ -1279,9 +1279,9 @@ void TeXOnePar(Buffer const & buf,
 				// do nothing at the end of child documents
 				if (maintext && buf.masterBuffer() != &buf)
 					break;
-				// end of main text
+				// end of main text: also insert a \clearpage (see #5386)
 				if (maintext) {
-					os << "\n\\end{CJK}\n";
+					os << "\n\\clearpage\n\\end{CJK}\n";
 				// end of an inset
 				} else
 					os << "\\end{CJK}";