M4: the list start reset runs once per edit and leaves plain paragraphs' levels alone

insert_model cleared the copied start with one set_para_lists call per newline, each scanning the whole text, and set_para_lists also zeroed the level of plain paragraphs. Add TextData::clear_list_starts (one pass, touches only start_offset, no-op when no paragraph has a start) and call it once over the created paragraphs, only when the text has a newline.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Model-Effort: Opus 5.5 drove; Opus 5.5 briefed; Sonnet 5.5 wrote at medium

crates/doc/src/text.rs

@@ -886,6 +886,25 @@ impl TextData {
             }
             pl.indent = pl.indent.min(MAX_LIST_INDENT);
         }
+        self.fold_paragraph_lists();
+    }
+
+    /// Give paragraphs `a..=b` (by index) no start number of their own, as an item made by Enter
+    /// has none. Touches nothing else: a plain paragraph keeps its level, a bullet its settings.
+    /// One pass over the range, and nothing at all when no paragraph carries a start.
+    pub fn clear_list_starts(&mut self, a: usize, b: usize) {
+        if self.paragraphs.iter().all(|p| p.start_offset == 0) {
+            return;
+        }
+        self.fill_paragraphs();
+        for pl in self.paragraphs.get_mut(a..=b).unwrap_or(&mut []) {
+            pl.start_offset = 0;
+        }
+        self.fold_paragraph_lists();
+    }
+
+    /// One style for all paragraphs with no nesting or start folds back into `list`.
+    fn fold_paragraph_lists(&mut self) {
         if let Some(first) = self.paragraphs.first().copied()
             && first == (ParaList { list: first.list, ..ParaList::default() })
             && self.paragraphs.iter().all(|p| *p == first)
@@ -1104,6 +1123,25 @@ mod case_tests {
         t.set_para_lists(0, 0, |p| p.list = P);
         assert_eq!(t.para_list(0), ParaList::default());
     }
+
+    #[test]
+    fn clear_list_starts_touches_only_starts() {
+        use ListStyle::{None as P, Numbered as N};
+        let mut t = TextData { characters: "a\nb\nc".into(), ..Default::default() };
+        t.paragraphs = vec![
+            ParaList { list: N, start_offset: 4, ..ParaList::default() },
+            ParaList { list: P, indent: 1, ..ParaList::default() },
+            ParaList { list: N, indent: 2, start_offset: 7, ..ParaList::default() },
+        ];
+        t.clear_list_starts(1, 2);
+        assert_eq!(t.para_list(0).start_offset, 4, "outside the range");
+        assert_eq!(t.para_list(1), ParaList { list: P, indent: 1, ..ParaList::default() }, "a plain paragraph keeps its level");
+        assert_eq!(t.para_list(2), ParaList { list: N, indent: 2, start_offset: 0, ..ParaList::default() });
+        // Out of range and no-start cases leave it alone.
+        t.clear_list_starts(5, 9);
+        t.clear_list_starts(0, 0);
+        assert_eq!(t.para_list(0).start_offset, 0);
+    }
 }
 
 #[cfg(test)]

crates/engine/src/canvas/textedit.rs

@@ -322,10 +322,9 @@ fn insert_model(s: &mut Session, text: &str, ime_commit: bool) -> Result<ImeInse
         // As measured in Figma (2026-11-11): an item made by Enter has no start of its own, so
         // unlisting the item before it makes it restart at 1 rather than at the copied start.
         // Every paragraph after the first that the insertion made loses the start it was copied.
-        for (i, c) in text.chars().enumerate() {
-            if c == '\n' {
-                t.set_para_lists(a + i + 1, a + i + 1, |pl| pl.start_offset = 0);
-            }
+        if text.contains('\n') {
+            let end = a + text.chars().count();
+            t.clear_list_starts(t.para_of(a) + 1, t.para_of(end));
         }
         te.caret = a + text.chars().count();
         te.anchor = te.caret;

crates/engine/src/text_tail_tests.rs

@@ -446,3 +446,39 @@ fn typing_a_creation_character_and_a_space_starts_a_list() {
     }
     assert_eq!((data(&s, t).characters.as_str(), lists(&s, t)), ("2) ", vec![(P, 0)]));
 }
+
+#[test]
+fn splitting_a_plain_paragraph_that_kept_its_level_keeps_the_level() {
+    // Backspace at a nested item's start unlists it but keeps indent 1; Enter inside it then
+    // splits it, and neither half loses the level (the start reset changes only starts).
+    let mut s = Session::new();
+    let t = text(&mut s, "ab\ncd");
+    let ps = vec![ParaList { list: N, start_offset: 4, ..ParaList::default() }, ParaList { list: N, indent: 1, ..ParaList::default() }];
+    if let figcraft_doc::NodeKind::Text(td) = &mut s.doc_mut().unwrap().get_mut(t).unwrap().kind {
+        td.paragraphs = ps;
+    }
+    begin(&mut s, t, Some(3), false).unwrap();
+    key(&mut s, "Backspace", false, false, false).unwrap();
+    assert_eq!(lists(&s, t), [(N, 0), (P, 1)], "unlisted, level kept");
+    set_caret(&mut s, 4, false).unwrap();
+    key(&mut s, "Enter", false, false, false).unwrap();
+    assert_eq!(data(&s, t).characters, "ab\nc\nd");
+    assert_eq!(lists(&s, t), [(N, 0), (P, 1), (P, 1)], "both halves keep indent 1");
+}
+
+#[test]
+fn a_many_line_paste_clears_every_created_start() {
+    let mut s = Session::new();
+    let t = text(&mut s, "x");
+    s.execute("node.set", &json!({"id": t, "props": {"characters": ""}})).unwrap();
+    begin(&mut s, t, Some(0), false).unwrap();
+    for c in "5. x".chars() {
+        insert(&mut s, &c.to_string()).unwrap();
+    }
+    let lines: Vec<String> = (0..2001).map(|i| format!("l{i}")).collect();
+    insert(&mut s, &lines.join("\n")).unwrap();
+    let d = data(&s, t);
+    assert_eq!(d.paragraph_count(), 2001);
+    assert_eq!(d.para_list(0).start_offset, 4, "the first item keeps its start");
+    assert!((1..d.paragraph_count()).all(|p| d.para_list(p).start_offset == 0 && d.para_list(p).list == N));
+}