M7: per-paragraph paragraph attributes

Each paragraph has its own alignment, indents, spacing, hyphenation, tab
stops and paragraph style (TextObject.paras; para stays paragraph 0's, so
old files and readers work). Layout, editing (split/merge), threads, the
Paragraph/Tabs/Paragraph Styles panels, ranged text.setStyle/setFormat/
tabs/paraStyle.apply, SVG export/import and the fuzzers follow.

crates/tools/src/text.rs

@@ -44,6 +44,10 @@ enum Mode {
 struct Typing {
     base: Vec<TextRun>,
     cur: Vec<TextRun>,
+    /// The first byte any edit of the session touched (the text before it is unchanged): the
+    /// previewed span starts no later, so Return at a paragraph's end splits that paragraph (its
+    /// style continues), not the next one.
+    lo: usize,
 }
 
 /// IME marked text inside the typing session, not yet committed.
@@ -174,9 +178,10 @@ impl TypeTool {
         if self.typing.is_none() {
             out.push(Action::Begin("Typing".into()));
         }
-        let ty = self.typing.get_or_insert_with(|| Typing { base: t.runs.clone(), cur: t.runs.clone() });
+        let ty = self.typing.get_or_insert_with(|| Typing { base: t.runs.clone(), cur: t.runs.clone(), lo: usize::MAX });
         let len = edit::runs_len(&ty.cur);
         let (a, b) = (self.caret.min(self.anchor).min(len), self.caret.max(self.anchor).min(len));
+        ty.lo = ty.lo.min(a);
         let caret = match &styled {
             Some(r) => edit::replace_range_styled(&mut ty.cur, a, b, r),
             None => edit::replace_range(&mut ty.cur, a, b, insert),
@@ -187,7 +192,7 @@ impl TypeTool {
         // One preview for the whole session: the changed span of base → cur, with its runs.
         let base: String = ty.base.iter().map(|r| r.text.as_str()).collect();
         let cur: String = ty.cur.iter().map(|r| r.text.as_str()).collect();
-        let (p, s) = common_affixes(&base, &cur);
+        let (p, s) = common_affixes(&base, &cur, ty.lo);
         let runs = edit::slice_runs(&ty.cur, p, cur.len() - s);
         let runs = if runs.is_empty() { json!([]) } else { serde_json::to_value(&runs).unwrap_or(json!([])) };
         out.push(Action::Preview("text.editRange".into(), json!({"id": id.0, "start": p, "end": base.len() - s, "runs": runs})));
@@ -332,11 +337,12 @@ fn char_range_to_bytes(s: &str, r: Range<usize>) -> Option<Range<usize>> {
     (a <= b).then_some(a..b)
 }
 
-/// Lengths of the common prefix and suffix of `a` and `b` (on char boundaries, not overlapping).
-fn common_affixes(a: &str, b: &str) -> (usize, usize) {
+/// Lengths of the common prefix (at most `max_prefix`) and suffix of `a` and `b` (on char
+/// boundaries, not overlapping).
+fn common_affixes(a: &str, b: &str, max_prefix: usize) -> (usize, usize) {
     let mut p = 0;
     for ((i, x), y) in a.char_indices().zip(b.chars()) {
-        if x != y {
+        if x != y || i + x.len_utf8() > max_prefix {
             break;
         }
         p = i + x.len_utf8();

crates/tools/src/text_tests.rs

@@ -221,10 +221,13 @@ fn select_all_and_styled_paste() {
 
 #[test]
 fn common_affixes_are_char_safe() {
-    assert_eq!(common_affixes("hello", "help"), (3, 0));
-    assert_eq!(common_affixes("aXb", "ab"), (1, 1));
-    assert_eq!(common_affixes("aaa", "aaaa"), (3, 0));
-    assert_eq!(common_affixes("é1", "è1"), (0, 1));
+    assert_eq!(common_affixes("hello", "help", usize::MAX), (3, 0));
+    assert_eq!(common_affixes("aXb", "ab", usize::MAX), (1, 1));
+    assert_eq!(common_affixes("aaa", "aaaa", usize::MAX), (3, 0));
+    assert_eq!(common_affixes("é1", "è1", usize::MAX), (0, 1));
+    // Return at the end of a paragraph: the span starts where the edit did (before the old break).
+    assert_eq!(common_affixes("abc\ndef", "abc\n\ndef", 3), (3, 4));
+    assert_eq!(common_affixes("abc\ndef", "abc\n\ndef", 4), (4, 3));
 }
 
 #[test]