fix: toc sticky auto-scroll

src/app/content.rs

@@ -54,7 +54,7 @@ impl App {
 
     pub(crate) fn reload(&mut self, ss: &SyntaxSet, themes: &ThemeSet) -> bool {
         self.reset_numkey_state();
-        self.reset_toc_scroll_mode();
+        self.clear_toc_scroll_state();
         let path = match &self.filepath {
             Some(p) => p,
             None => return false,
@@ -130,7 +130,7 @@ impl App {
         self.invalidate_theme_preview_cache();
         self.store_current_theme_preview_from(&parsed.lines, &parsed.toc);
         self.replace_content(parsed);
-        self.reset_toc_scroll_mode();
+        self.clear_toc_scroll_state();
         true
     }
 

src/app/mod.rs

@@ -50,10 +50,18 @@ pub(crate) use theme_picker::ThemePickerState;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub(crate) enum TocScrollMode {
-    Auto,
+    Auto(usize),
     Manual(usize),
 }
 
+impl TocScrollMode {
+    pub(crate) fn offset(self) -> usize {
+        match self {
+            TocScrollMode::Auto(o) | TocScrollMode::Manual(o) => o,
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct StatusCacheKey {
     pct: u16,
@@ -260,7 +268,7 @@ impl App {
             toc_active_idx: None,
             toc_active_display_idx: None,
             hovered_toc_idx: None,
-            toc_scroll_mode: TocScrollMode::Auto,
+            toc_scroll_mode: TocScrollMode::Auto(0),
             toc_scroll_hint_dismissed: false,
             toc_active_pin: None,
             status_line: Line::default(),
@@ -539,6 +547,7 @@ impl App {
     pub(crate) fn refresh_toc_cache(&mut self) {
         let active_idx = self.active_toc_index();
         if self.toc_active_idx == active_idx && !self.toc_display_lines.is_empty() {
+            self.adjust_auto_toc_offset();
             return;
         }
 
@@ -568,6 +577,30 @@ impl App {
         self.toc_display_lines = lines;
         self.toc_display_entries = entries;
         self.toc_active_display_idx = active_display_idx;
+        self.adjust_auto_toc_offset();
+    }
+
+    fn adjust_auto_toc_offset(&mut self) {
+        let TocScrollMode::Auto(current_offset) = self.toc_scroll_mode else {
+            return;
+        };
+        let (Some(active), Some(area)) = (self.toc_active_display_idx, self.toc_list_area) else {
+            return;
+        };
+        let list_height = area.height as usize;
+        if list_height == 0 {
+            return;
+        }
+        let max_offset = self.max_toc_scroll_offset(area.height);
+
+        let new_offset = if active + 1 >= current_offset + list_height {
+            (active + 2).saturating_sub(list_height)
+        } else if active <= current_offset {
+            active.saturating_sub(1)
+        } else {
+            current_offset
+        };
+        self.toc_scroll_mode = TocScrollMode::Auto(new_offset.min(max_offset));
     }
 
     pub(crate) fn refresh_status_cache(&mut self, pct: u16) {

src/app/navigation.rs

@@ -177,31 +177,24 @@ impl App {
             .is_some_and(|area| self.toc_overflows(area.height))
     }
 
-    fn max_toc_scroll_offset(&self, list_height: u16) -> usize {
+    pub(super) fn max_toc_scroll_offset(&self, list_height: u16) -> usize {
         (self.toc_display_lines.len() + 1).saturating_sub(list_height as usize)
     }
 
     pub(crate) fn toc_scroll_offset(&self, list_height: u16) -> usize {
         let max_offset = self.max_toc_scroll_offset(list_height);
-        let list_height = list_height as usize;
-        match self.toc_scroll_mode {
-            TocScrollMode::Manual(offset) => offset.min(max_offset),
-            TocScrollMode::Auto => {
-                let Some(active_display_idx) = self.toc_active_display_idx else {
-                    return 0;
-                };
-                let mut offset = 0usize;
-                if active_display_idx + 1 >= list_height {
-                    offset = active_display_idx + 2 - list_height;
-                }
-                offset.min(max_offset)
-            }
-        }
+        self.toc_scroll_mode.offset().min(max_offset)
     }
 
     pub(crate) fn reset_toc_scroll_mode(&mut self) {
-        self.toc_scroll_mode = TocScrollMode::Auto;
+        self.toc_scroll_mode = TocScrollMode::Auto(self.toc_scroll_mode.offset());
+        self.toc_active_pin = None;
+    }
+
+    pub(crate) fn clear_toc_scroll_state(&mut self) {
+        self.toc_scroll_mode = TocScrollMode::Auto(0);
         self.toc_active_pin = None;
+        self.toc_scroll_hint_dismissed = false;
     }
 
     pub(crate) fn scroll_toc_down(&mut self, n: usize) {

src/tests/toc.rs

@@ -220,7 +220,7 @@ fn make_app_with_overflowing_toc() -> App {
 #[test]
 fn toc_scroll_mode_default_is_auto() {
     let app = make_app_with_toc(10, 15, toc(&[(2, 0)]));
-    assert_eq!(app.toc_scroll_mode(), TocScrollMode::Auto);
+    assert_eq!(app.toc_scroll_mode(), TocScrollMode::Auto(0));
     assert!(!app.is_toc_scroll_hint_dismissed());
 }
 
@@ -279,7 +279,7 @@ fn content_scroll_resets_toc_scroll_mode_to_auto() {
     app.scroll_toc_down(5);
     assert!(matches!(app.toc_scroll_mode(), TocScrollMode::Manual(_)));
     app.scroll_down(1);
-    assert_eq!(app.toc_scroll_mode(), TocScrollMode::Auto);
+    assert!(matches!(app.toc_scroll_mode(), TocScrollMode::Auto(_)));
 }
 
 #[test]