[LyX/master] Revert "XHTML: remove <section> support."

[Thibaut Cuvelier]

commit 15dbc2b9ab0765b950ebe57300178fa4ec59f3cd
Author: Thibaut Cuvelier <tcuvelier at lyx.org>
Date:   Wed Sep 16 03:37:46 2020 +0200

    Revert "XHTML: remove <section> support."
    
    This reverts commit 52216b73
---
 src/output_xhtml.cpp |   37 +++++++++++++++++++++++++++++++++++++
 1 files changed, 37 insertions(+), 0 deletions(-)

diff --git a/src/output_xhtml.cpp b/src/output_xhtml.cpp
index 99d425e..f8cd53e 100644
--- a/src/output_xhtml.cpp
+++ b/src/output_xhtml.cpp
@@ -590,6 +590,7 @@ void xhtmlParagraphs(Text const & text,
 	ParagraphList::const_iterator const pend =
 		(epit == (int) paragraphs.size()) ?
 			paragraphs.end() : paragraphs.iterator_at(epit);
+	std::stack<int> headerLevels;
 
 	while (bpit < epit) {
 		ParagraphList::const_iterator par = paragraphs.iterator_at(bpit);
@@ -608,6 +609,33 @@ void xhtmlParagraphs(Text const & text,
 		ParagraphList::const_iterator const lastpar = par;
 		ParagraphList::const_iterator send;
 
+		// Think about adding <section> and/or </section>s.
+		if (style.category() == from_utf8("Sectioning")) {
+			int level = style.toclevel;
+
+			// Need to close a previous section if it has the same level or a higher one (close <section> if opening a
+			// <h2> after a <h2>, <h3>, <h4>, <h5> or <h6>). More examples:
+			//   - current: h2; back: h1; do not close any <section>
+			//   - current: h1; back: h2; close two <section> (first the <h2>, then the <h1>, so a new <h1> can come)
+			while (!headerLevels.empty() && level <= headerLevels.top()) {
+				// Output the tag only if it corresponds to a legit section.
+				int stackLevel = headerLevels.top();
+				if (stackLevel != Layout::NOT_IN_TOC && level > 1) { // <h1> is the document title.
+					xs << xml::EndTag("section");
+					xs << xml::CR();
+				}
+				headerLevels.pop();
+			}
+
+			// Open the new section: first push it onto the stack, then output it in XHTML.
+			headerLevels.push(level);
+			// Some sectioning-like elements should not be output (such as FrontMatter).
+			if (level != Layout::NOT_IN_TOC && level > 1) { // <h1> is the document title.
+				xs << xml::StartTag("section");
+				xs << xml::CR();
+			}
+		}
+
 		switch (style.latextype) {
 		case LATEX_COMMAND: {
 			// The files with which we are working never have more than
@@ -644,6 +672,15 @@ void xhtmlParagraphs(Text const & text,
 		}
 		bpit += distance(lastpar, par);
 	}
+
+	// If need be, close <section>s, but only at the end of the document (otherwise, dealt with at the beginning
+	// of the loop).
+	while (!headerLevels.empty() && headerLevels.top() > Layout::NOT_IN_TOC) {
+		docstring tag = from_utf8("</section>");
+		headerLevels.pop();
+		xs << XMLStream::ESCAPE_NONE << tag;
+		xs << xml::CR();
+	}
 }