fix: toc highlight on last header

src/app/mod.rs

@@ -331,13 +331,15 @@ impl App {
 
     pub(crate) fn active_toc_index(&self) -> Option<usize> {
         let hide_single_h1 = should_hide_single_h1(&self.toc);
+        let is_visible = |entry: &&TocEntry| !(hide_single_h1 && entry.level == 1);
+
         let mut first_visible = None;
         let mut active = None;
         for (idx, entry) in self
             .toc
             .iter()
             .enumerate()
-            .filter(|(_, entry)| !(hide_single_h1 && entry.level == 1))
+            .filter(|(_, entry)| is_visible(entry))
         {
             if first_visible.is_none() {
                 first_visible = Some((idx, entry.line));
@@ -348,6 +350,17 @@ impl App {
             active = Some(idx);
         }
 
+        let max = self.max_scroll();
+        if max > 0 && self.scroll >= max {
+            let last_visible = self
+                .toc
+                .iter()
+                .enumerate()
+                .rfind(|(_, entry)| is_visible(entry))
+                .map(|(idx, _)| idx);
+            active = last_visible;
+        }
+
         let (first_idx, first_line) = first_visible?;
         if self.scroll < first_line {
             Some(first_idx)

src/tests/markdown_blocks.rs

@@ -54,33 +54,14 @@ fn long_blockquotes_wrap_into_multiple_prefixed_lines() {
 }
 
 #[test]
-fn toc_only_includes_first_two_heading_levels() {
-    let (ss, theme) = test_assets();
-    let (_, toc, _) = parse_markdown(
-        "# One\n## Two\n### Three\n#### Four\n",
-        &ss,
-        &theme,
-        &test_md_theme(),
-        false,
-    );
-
-    assert_eq!(toc.len(), 3);
-    assert_eq!(toc[0].level, 1);
-    assert_eq!(toc[1].level, 2);
-    assert_eq!(toc[2].level, 3);
-}
-
-#[test]
-fn frontmatter_is_ignored_in_preview_and_toc() {
+fn frontmatter_is_ignored_in_preview() {
     let (ss, theme) = test_assets();
     let src = "---\ntitle: Demo\nowner: me\n---\n# Visible\nBody\n";
-    let (lines, toc, _) = parse_markdown(src, &ss, &theme, &test_md_theme(), false);
+    let (lines, _, _) = parse_markdown(src, &ss, &theme, &test_md_theme(), false);
     let rendered = rendered_non_empty_lines(&lines);
 
     assert!(!rendered.iter().any(|line| line.contains("title: Demo")));
     assert!(rendered.iter().any(|line| line.contains("Visible")));
-    assert_eq!(toc.len(), 1);
-    assert_eq!(toc[0].title, "Visible");
 }
 
 #[test]
@@ -121,58 +102,3 @@ fn rules_use_render_width_without_extra_blank_after() {
     let rule_idx = rendered.iter().position(|line| line == rule).unwrap();
     assert_eq!(rendered[rule_idx + 1], "Beta");
 }
-
-#[test]
-fn toc_hides_single_h1_when_h2_entries_exist() {
-    let toc = vec![
-        TocEntry {
-            level: 1,
-            title: "Doc Title".to_string(),
-            line: 0,
-        },
-        TocEntry {
-            level: 2,
-            title: "Install".to_string(),
-            line: 10,
-        },
-    ];
-
-    assert!(should_hide_single_h1(&toc));
-    assert_eq!(toc_display_level(2, true, false), 1);
-    assert_eq!(toc_display_level(3, true, false), 2);
-}
-
-#[test]
-fn toc_keeps_single_h1_when_no_h2_entries_exist() {
-    let toc = vec![TocEntry {
-        level: 1,
-        title: "Doc Title".to_string(),
-        line: 0,
-    }];
-
-    assert!(!should_hide_single_h1(&toc));
-}
-
-#[test]
-fn toc_promotes_h2_when_document_has_no_h1() {
-    let toc = vec![
-        TocEntry {
-            level: 2,
-            title: "Build & install".to_string(),
-            line: 0,
-        },
-        TocEntry {
-            level: 3,
-            title: "Android".to_string(),
-            line: 4,
-        },
-    ];
-
-    assert!(should_promote_h2_when_no_h1(&toc));
-    assert_eq!(toc_display_level(2, false, true), 1);
-    assert_eq!(toc_display_level(3, false, true), 2);
-    let normalized = normalize_toc(toc);
-    assert_eq!(normalized.len(), 2);
-    assert_eq!(normalized[0].level, 2);
-    assert_eq!(normalized[1].level, 3);
-}

src/tests/mod.rs

@@ -24,6 +24,7 @@ mod markdown_lists;
 mod markdown_tables;
 mod render;
 mod theme;
+mod toc;
 mod update;
 
 pub(super) static THEME_TEST_MUTEX: Mutex<()> = Mutex::new(());

src/tests/toc.rs

@@ -0,0 +1,151 @@
+use super::{test_assets, test_md_theme};
+use crate::app::App;
+use crate::markdown::parse_markdown;
+use crate::*;
+use ratatui::layout::Rect;
+
+fn toc(entries: &[(u8, usize)]) -> Vec<TocEntry> {
+    entries
+        .iter()
+        .enumerate()
+        .map(|(i, (level, line))| TocEntry {
+            level: *level,
+            title: format!("Section {}", i + 1),
+            line: *line,
+        })
+        .collect()
+}
+
+fn make_app_with_toc(total_lines: usize, viewport_height: u16, toc: Vec<TocEntry>) -> App {
+    let (ss, theme) = test_assets();
+    let md = (0..total_lines)
+        .map(|_| "line")
+        .collect::<Vec<_>>()
+        .join("\n");
+    let (lines, _, _) = parse_markdown(&md, &ss, &theme, &test_md_theme(), false);
+    let mut app = App::new(lines, toc, "test".to_string(), false, false, None, None);
+    app.content_area = Rect::new(0, 0, 80, viewport_height);
+    app
+}
+
+#[test]
+fn active_toc_highlights_last_header_when_short_section_at_bottom() {
+    let mut app = make_app_with_toc(100, 15, toc(&[(2, 0), (2, 30), (2, 70), (2, 95)]));
+    app.scroll_bottom();
+    assert_eq!(app.active_toc_index(), Some(3));
+}
+
+#[test]
+fn active_toc_unchanged_when_document_fits_in_viewport() {
+    let mut app = make_app_with_toc(10, 20, toc(&[(2, 0), (2, 5)]));
+    app.scroll_bottom();
+    assert_eq!(app.active_toc_index(), Some(0));
+}
+
+#[test]
+fn active_toc_last_header_with_long_section_uses_existing_logic() {
+    let mut app = make_app_with_toc(100, 15, toc(&[(2, 0), (2, 30), (2, 50)]));
+    app.scroll_bottom();
+    assert_eq!(app.active_toc_index(), Some(2));
+}
+
+#[test]
+fn active_toc_intermediate_header() {
+    let mut app = make_app_with_toc(100, 15, toc(&[(2, 0), (2, 30), (2, 70)]));
+    app.scroll = 40;
+    assert_eq!(app.active_toc_index(), Some(1));
+}
+
+#[test]
+fn active_toc_empty_toc_returns_none() {
+    let app = make_app_with_toc(50, 15, vec![]);
+    assert_eq!(app.active_toc_index(), None);
+}
+
+#[test]
+fn active_toc_single_header() {
+    let app = make_app_with_toc(50, 15, toc(&[(2, 0)]));
+    assert_eq!(app.active_toc_index(), Some(0));
+}
+
+#[test]
+fn toc_only_includes_first_two_heading_levels() {
+    let (ss, theme) = test_assets();
+    let (_, toc, _) = parse_markdown(
+        "# One\n## Two\n### Three\n#### Four\n",
+        &ss,
+        &theme,
+        &test_md_theme(),
+        false,
+    );
+
+    assert_eq!(toc.len(), 3);
+    assert_eq!(toc[0].level, 1);
+    assert_eq!(toc[1].level, 2);
+    assert_eq!(toc[2].level, 3);
+}
+
+#[test]
+fn frontmatter_is_ignored_in_toc() {
+    let (ss, theme) = test_assets();
+    let src = "---\ntitle: Demo\nowner: me\n---\n# Visible\nBody\n";
+    let (_, toc, _) = parse_markdown(src, &ss, &theme, &test_md_theme(), false);
+
+    assert_eq!(toc.len(), 1);
+    assert_eq!(toc[0].title, "Visible");
+}
+
+#[test]
+fn toc_hides_single_h1_when_h2_entries_exist() {
+    let toc = vec![
+        TocEntry {
+            level: 1,
+            title: "Doc Title".to_string(),
+            line: 0,
+        },
+        TocEntry {
+            level: 2,
+            title: "Install".to_string(),
+            line: 10,
+        },
+    ];
+
+    assert!(should_hide_single_h1(&toc));
+    assert_eq!(toc_display_level(2, true, false), 1);
+    assert_eq!(toc_display_level(3, true, false), 2);
+}
+
+#[test]
+fn toc_keeps_single_h1_when_no_h2_entries_exist() {
+    let toc = vec![TocEntry {
+        level: 1,
+        title: "Doc Title".to_string(),
+        line: 0,
+    }];
+
+    assert!(!should_hide_single_h1(&toc));
+}
+
+#[test]
+fn toc_promotes_h2_when_document_has_no_h1() {
+    let toc = vec![
+        TocEntry {
+            level: 2,
+            title: "Build & install".to_string(),
+            line: 0,
+        },
+        TocEntry {
+            level: 3,
+            title: "Android".to_string(),
+            line: 4,
+        },
+    ];
+
+    assert!(should_promote_h2_when_no_h1(&toc));
+    assert_eq!(toc_display_level(2, false, true), 1);
+    assert_eq!(toc_display_level(3, false, true), 2);
+    let normalized = normalize_toc(toc);
+    assert_eq!(normalized.len(), 2);
+    assert_eq!(normalized[0].level, 2);
+    assert_eq!(normalized[1].level, 3);
+}