From 5ab84500fed60deb67a3ef451d20fb765ad86a18 Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sun, 11 Oct 2026 11:39:36 +0200
Subject: [PATCH] Show the first parent page behind single-sided pages (#429)

---
 crates/doc/src/build.rs  | 17 ++++++++++++++++-
 crates/tools/src/snap.rs | 15 +++++++++++++--
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/crates/doc/src/build.rs b/crates/doc/src/build.rs
index c274f213..e31dc3c7 100644
--- a/crates/doc/src/build.rs
+++ b/crates/doc/src/build.rs
@@ -549,7 +549,9 @@ impl Document {
         let pi = self.parent_index(page.parent?)?;
         let ps = &self.parents[pi];
         // A parent spread can come without pages from a file.
-        let idx = if ps.pages.len() >= 2 && page.side == PageSide::Left { 0 } else { ps.pages.len().checked_sub(1)? };
+        // A single-sided page takes the parent's first page, like a left-hand page.
+        let first = ps.pages.len() >= 2 && matches!(page.side, PageSide::Left | PageSide::Single);
+        let idx = if first { 0 } else { ps.pages.len().checked_sub(1)? };
         Some((pi, idx))
     }
 }
@@ -584,6 +586,19 @@ mod tests {
         assert_eq!(d.parent_page_for(1), None);
     }
 
+    /// A single-sided page applying a two-page parent shows the parent's first page.
+    #[test]
+    fn single_sided_page_shows_first_parent_page() {
+        let mut d = Document::new(&NewDocument { pages: 2, facing_pages: false, ..Default::default() });
+        let extra = d.parents[0].pages[0].clone();
+        Arc::make_mut(&mut d.parents[0]).pages.push(extra);
+        assert_eq!(d.parents[0].pages.len(), 2);
+        assert_eq!(d.parent_page_for(0), Some((0, 0)));
+        assert_eq!(d.parent_page_for(1), Some((0, 0)));
+        Arc::make_mut(&mut d.parents[0]).pages.clear();
+        assert_eq!(d.parent_page_for(0), None);
+    }
+
     #[test]
     fn non_facing_one_page_per_spread() {
         let d = Document::new(&NewDocument { pages: 3, facing_pages: false, ..Default::default() });
diff --git a/crates/tools/src/snap.rs b/crates/tools/src/snap.rs
index 6d5c8f6a..50c95f66 100644
--- a/crates/tools/src/snap.rs
+++ b/crates/tools/src/snap.rs
@@ -1291,9 +1291,10 @@ fn offer_parent_chain(
     offer_parent_chain(best, cx, req, flags, axis, tol, doc_x, doc_side, overridden, next_index, seen);
 }
 
-/// Left page of a facing parent, otherwise the last page. Same choice as `Document::parent_page_for`.
+/// First page of a multi-page parent for a left-hand or single-sided page, otherwise the last page.
+/// Same choice as `Document::parent_page_for`.
 fn shown_parent_page_index(page_count: usize, side: PageSide) -> Option<usize> {
-    if page_count >= 2 && side == PageSide::Left { Some(0) } else { page_count.checked_sub(1) }
+    if page_count >= 2 && matches!(side, PageSide::Left | PageSide::Single) { Some(0) } else { page_count.checked_sub(1) }
 }
 
 fn shift_x(r: Rect, dx: f64) -> Rect {
@@ -1444,6 +1445,16 @@ mod tests {
 
     use super::*;
 
+    /// Snapping offers the parent page the canvas shows: a single-sided page uses a two-page parent's first page.
+    #[test]
+    fn shown_parent_page_matches_page_side() {
+        assert_eq!(shown_parent_page_index(2, PageSide::Single), Some(0));
+        assert_eq!(shown_parent_page_index(2, PageSide::Left), Some(0));
+        assert_eq!(shown_parent_page_index(2, PageSide::Right), Some(1));
+        assert_eq!(shown_parent_page_index(1, PageSide::Single), Some(0));
+        assert_eq!(shown_parent_page_index(0, PageSide::Single), None);
+    }
+
     // The factory flags are const. Pin them anyway.
     #[test]
     #[allow(clippy::assertions_on_constants)]