From 91fad115285b802b095cb240b5706754d102562f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matilda=20Bergstr=C3=B6m?=
 <matildabergstrom@matildas-mbp-3.students.netlogon.liu.se>
Date: Thu, 11 Sep 2025 09:49:34 +0200
Subject: [PATCH] latex: remove blank lines inside math blocks (fixes #3466)

---
 rslib/src/latex.rs | 48 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/rslib/src/latex.rs b/rslib/src/latex.rs
index 02056b721..a72a376bf 100644
--- a/rslib/src/latex.rs
+++ b/rslib/src/latex.rs
@@ -64,18 +64,24 @@ pub fn extract_latex(text: &str, svg: bool) -> (Cow<'_, str>, Vec<ExtractedLatex
     let mut extracted = vec![];
 
     let new_text = LATEX.replace_all(text, |caps: &Captures| {
+        // [latex] blocks preserve newlines between <br>/<div>
+        // [$] and [$$] blocks remove <br>/<div> completely so that
+        // no blank lines are introduced inside math environments.
         let latex = match (caps.get(1), caps.get(2), caps.get(3)) {
-            (Some(m), _, _) => m.as_str().into(),
-            (_, Some(m), _) => format!("${}$", m.as_str()),
-            (_, _, Some(m)) => format!(r"\begin{{displaymath}}{}\end{{displaymath}}", m.as_str()),
+            (Some(m), _, _) => strip_html_for_latex(m.as_str()).into(),
+            (_, Some(m), _) => format!("${}$", strip_html_for_latex_math(m.as_str())),
+            (_, _, Some(m)) => format!(
+                r"\begin{{displaymath}}{}\end{{displaymath}}",
+                strip_html_for_latex_math(m.as_str())
+            ),
             _ => unreachable!(),
         };
-        let latex_text = strip_html_for_latex(&latex);
-        let fname = fname_for_latex(&latex_text, svg);
-        let img_link = image_link_for_fname(&latex_text, &fname);
+
+        let fname = fname_for_latex(&latex, svg);
+        let img_link = image_link_for_fname(&latex, &fname);
         extracted.push(ExtractedLatex {
             fname,
-            latex: latex_text.into(),
+            latex: latex.into(),
         });
 
         img_link
@@ -96,6 +102,20 @@ fn strip_html_for_latex(html: &str) -> Cow<'_, str> {
     out
 }
 
+/// Removes HTML breaks (<br>, <div>) from math blocks instead of
+/// converting them to newlines. This prevents LaTeX environments
+/// from being broken by unintended blank lines.
+fn strip_html_for_latex_math(html: &str) -> Cow<'_, str> {
+    let mut out: Cow<str> = html.into();
+    if let Cow::Owned(o) = LATEX_NEWLINES.replace_all(html, "") {
+        out = o.into();
+    }
+    if let Cow::Owned(o) = strip_html(out.as_ref()) {
+        out = o.into();
+    }
+    out
+}
+
 fn fname_for_latex(latex: &str, svg: bool) -> String {
     let ext = if svg { "svg" } else { "png" };
     let csum = hex::encode(sha1_of_data(latex.as_bytes()));
@@ -146,4 +166,18 @@ mod test {
             }]
         );
     }
+
+    /// Ensures that math blocks do not contain unintended blank lines
+    /// when <br> or <div> elements are present in the HTML.
+    #[test]
+    fn no_blank_lines_in_math() {
+        let input = "[$$]\\begin{tikzcd}x & y\\end{tikzcd}[/$$]";
+        let (_, extracts) = extract_latex(input, false);
+        let extracted = &extracts[0].latex;
+
+        assert!(
+            !extracted.contains("\n\n"),
+            "Should not contain blank lines inside math"
+        );
+    }
 }