Adjust the baseline of root radicand frame (#4736)

crates/typst/src/math/root.rs

@@ -70,8 +70,20 @@ fn layout(
     let raise_factor = percent!(ctx, radical_degree_bottom_raise_percent);
 
     // Layout radicand.
+    let radicand = {
         let cramped = style_cramped();
-    let radicand = ctx.layout_into_frame(radicand, styles.chain(&cramped))?;
+        let styles = styles.chain(&cramped);
+        let run = ctx.layout_into_run(radicand, styles)?;
+        let multiline = run.is_multiline();
+        let mut radicand = run.into_fragment(ctx, styles).into_frame();
+        if multiline {
+            // Align the frame center line with the math axis.
+            radicand.set_baseline(
+                radicand.height() / 2.0 + scaled!(ctx, styles, axis_height),
+            );
+        }
+        radicand
+    };
 
     // Layout root symbol.
     let target = radicand.height() + thickness + gap;

crates/typst/src/math/row.rs

@@ -353,7 +353,7 @@ impl MathRun {
         items
     }
 
-    fn is_multiline(&self) -> bool {
+    pub fn is_multiline(&self) -> bool {
         self.iter().any(|frag| matches!(frag, MathFragment::Linebreak))
     }
 }

tests/suite/math/root.typ

@@ -30,6 +30,7 @@ $ sqrt([|x|]^2 + [|y|]^2) < [|z|] $
 $ v = sqrt((1/2) / (4/5))
    = root(3, (1/2/3) / (4/5/6))
    = root(4, ((1/2) / (3/4)) / ((1/2) / (3/4))) $
+$ v = sqrt(a +\ b) $
 
 --- math-root-large-index ---
 // Test large index.