Layout: the caret on the empty line after a trailing line break follows the paragraph's alignment (#4293)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/layout/src/para/lines.rs

@@ -584,6 +584,13 @@ impl<'e> Breaker<'e> {
             (LineSpacing::AtLeast(v), None) => natural.max(v),
             (LineSpacing::Exactly(v), _) => v,
         };
+        // The empty line's caret follows the alignment, as an empty paragraph's does.
+        let slack = (self.base_right - rp.indent_left).max(0.0);
+        let shift = match rp.align {
+            Align::Center => slack / 2.0,
+            Align::Right => slack,
+            _ => 0.0,
+        };
         let (width, x0) = (self.width, rp.indent_left);
         let (x0, left, right) = if self.rtl_para { (width - x0, width - self.base_right, width - x0) } else { (x0, x0, self.base_right) };
         self.lines.push(Line {
@@ -594,7 +601,7 @@ impl<'e> Breaker<'e> {
             text_desc: desc,
             c0: self.n,
             c1: self.n,
-            xs: vec![x0],
+            xs: vec![if self.rtl_para { x0 - shift } else { x0 + shift }],
             leaders: Vec::new(),
             end: LineEnd::Para,
             start: pl.text_len,

crates/layout/src/tests.rs

@@ -1422,6 +1422,32 @@ fn center_and_right_alignment() {
     assert!((x2 - 540.0).abs() < 1.0, "{x2}");
 }
 
+#[test]
+fn caret_on_an_empty_line_after_a_break_follows_alignment() {
+    // "BAB I", Shift+Enter: the empty line's caret sits where its alignment puts it (#4283).
+    let text = format!("BAB I{}", wordcraft_doc::para::LINE_BREAK);
+    for rtl in [false, true] {
+        for (align, ltr_x) in [(Align::Left, 72.0), (Align::Center, 306.0), (Align::Right, 540.0), (Align::Justify, 72.0)] {
+            let mut d = Document::new();
+            d.body = vec![wordcraft_doc::para_block(wordcraft_doc::Paragraph::with_text(&text, Default::default()))];
+            let at = Pos::body(0, 0);
+            d.format_paragraphs(&at, &at, &|p: &mut ParaProps| {
+                p.align = Some(align);
+                p.bidi = Some(rtl);
+            })
+            .unwrap();
+            let expected = if rtl { 612.0 - ltr_x } else { ltr_x };
+            let x = lay(&d).caret(&Pos::body(0, text.len())).unwrap().x;
+            assert!((x - expected).abs() < 1.0, "{align:?} rtl={rtl}: {x}, expected {expected}");
+        }
+    }
+    // An empty centred paragraph without a break was already centred.
+    let mut d = Document::from_text("");
+    d.format_paragraphs(&Pos::body(0, 0), &Pos::body(0, 0), &|p| p.align = Some(Align::Center)).unwrap();
+    let x = lay(&d).caret(&Pos::body(0, 0)).unwrap().x;
+    assert!((x - 306.0).abs() < 1.0, "{x}");
+}
+
 #[test]
 fn tabs_land_on_default_stops() {
     let d = Document::from_text("a\tb");