Allow a function as an argument to size in accent

The short fall is now only applied in the default for `accent`
(`x => x - 0.5em`). This is the reason for the updated tests.

crates/typst-layout/src/math/accent.rs

@@ -1,13 +1,10 @@
 use typst_library::diag::SourceResult;
 use typst_library::foundations::{Packed, StyleChain};
-use typst_library::layout::{Em, Frame, Point, Size};
+use typst_library::layout::{Abs, Frame, Point, Size};
 use typst_library::math::{Accent, AccentElem};
 
 use super::{style_cramped, FrameFragment, GlyphFragment, MathContext, MathFragment};
 
-/// How much the accent can be shorter than the base.
-const ACCENT_SHORT_FALL: Em = Em::new(0.5);
-
 /// Lays out an [`AccentElem`].
 #[typst_macros::time(name = "math.accent", span = elem.span())]
 pub fn layout_accent(
@@ -29,7 +26,7 @@ pub fn layout_accent(
     let base_class = base.class();
     let base_attach = base.accent_attach();
 
-    let width = elem.size(styles).relative_to(base.width());
+    let width = elem.size(styles).resolve(ctx.engine, styles, base.width())?;
 
     let Accent(c) = elem.accent;
     let mut glyph = GlyphFragment::new(ctx, styles, c, elem.span());
@@ -42,8 +39,7 @@ pub fn layout_accent(
 
     // Forcing the accent to be at least as large as the base makes it too
     // wide in many case.
-    let short_fall = ACCENT_SHORT_FALL.at(glyph.font_size);
+    let variant = glyph.stretch_horizontal(ctx, width, Abs::zero());
-    let variant = glyph.stretch_horizontal(ctx, width, short_fall);
     let accent = variant.frame;
     let accent_attach = variant.accent_attach;
 

crates/typst-library/src/math/accent.rs

@@ -1,7 +1,19 @@
 use crate::diag::bail;
-use crate::foundations::{cast, elem, func, Content, NativeElement, SymbolElem};
+use crate::foundations::{
-use crate::layout::{Length, Rel};
+    cast, elem, func, Content, NativeElement, NativeFunc, SymbolElem,
-use crate::math::Mathy;
+};
+use crate::layout::{Em, Length, Ratio, Rel};
+use crate::math::{Mathy, StretchSize};
+
+const ACCENT_SHORT_FALL: Em = Em::new(-0.5);
+
+#[func(name = "x => x - 0.5em")]
+const fn default_accent_size(base: Length) -> Rel {
+    Rel {
+        rel: Ratio::zero(),
+        abs: Length { abs: base.abs, em: ACCENT_SHORT_FALL },
+    }
+}
 
 /// Attaches an accent to a base.
 ///
@@ -52,12 +64,14 @@ pub struct AccentElem {
 
     /// The size of the accent, relative to the width of the base.
     ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
+    ///
     /// ```example
     /// $dash(A, size: #150%)$
     /// ```
-    #[resolve]
+    #[default(<default_accent_size>::data().into())]
-    #[default(Rel::one())]
+    pub size: StretchSize,
-    pub size: Rel<Length>,
 
     /// Whether to remove the dot on top of lowercase i and j when adding a top
     /// accent.
@@ -116,8 +130,11 @@ macro_rules! accents {
                 /// The base to which the accent is applied.
                 base: Content,
                 /// The size of the accent, relative to the width of the base.
+                ///
+                /// See the [stretch documentation]($math.stretch.size) for
+                /// more information on sizes.
                 #[named]
-                size: Option<Rel<Length>>,
+                size: Option<StretchSize>,
                 /// Whether to remove the dot on top of lowercase i and j when
                 /// adding a top accent.
                 #[named]

tests/suite/math/accent.typ

@@ -30,12 +30,18 @@ $ tilde(integral), tilde(integral)_a^b, tilde(integral_a^b) $
 
 --- math-accent-sized ---
 // Test accent size.
-$tilde(sum), tilde(sum, size: #50%), accent(H, hat, size: #200%)$
+$tilde(sum), tilde(sum, size: #25%), accent(H, hat, size: #125%)$
 
 --- math-accent-sized-script ---
 // Test accent size in script size.
 $tilde(U, size: #1.1em), x^tilde(U, size: #1.1em), sscript(tilde(U, size: #1.1em))$
 
+--- math-accent-sized-function ---
+// Test accent size with a function.
+$dash(A) arrow(I) hat(L)$ \
+#set math.accent(size: x => x - 0.1em)
+$dash(A) arrow(I) hat(L)$
+
 --- math-accent-dotless ---
 // Test dotless glyph variants.
 #let test(c) = $grave(#c), acute(sans(#c)), hat(frak(#c)), tilde(mono(#c)),