Fix auto hanging-indent for centered headings (#6839)

crates/typst-layout/src/rules.rs

@@ -10,11 +10,11 @@ use typst_library::foundations::{
 };
 use typst_library::introspection::{Counter, Locator, LocatorLink};
 use typst_library::layout::{
-    Abs, AlignElem, Alignment, Axes, BlockBody, BlockElem, ColumnsElem, Em, GridCell,
-    GridChild, GridElem, GridItem, HAlignment, HElem, HideElem, InlineElem, LayoutElem,
-    Length, MoveElem, OuterVAlignment, PadElem, PlaceElem, PlacementScope, Region, Rel,
-    RepeatElem, RotateElem, ScaleElem, Sides, Size, Sizing, SkewElem, Spacing,
-    StackChild, StackElem, TrackSizings, VElem,
+    Abs, AlignElem, Alignment, Axes, BlockBody, BlockElem, ColumnsElem, Em,
+    FixedAlignment, GridCell, GridChild, GridElem, GridItem, HAlignment, HElem, HideElem,
+    InlineElem, LayoutElem, Length, MoveElem, OuterVAlignment, PadElem, PlaceElem,
+    PlacementScope, Region, Rel, RepeatElem, RotateElem, ScaleElem, Sides, Size, Sizing,
+    SkewElem, Spacing, StackChild, StackElem, TrackSizings, VElem,
 };
 use typst_library::math::EquationElem;
 use typst_library::model::{
@@ -240,8 +240,9 @@ const HEADING_RULE: ShowFn<HeadingElem> = |elem, engine, styles| {
         let numbering = Counter::of(HeadingElem::ELEM)
             .display_at_loc(engine, location, styles, numbering)?
             .spanned(span);
+        let align = styles.resolve(AlignElem::alignment);
 
-        if hanging_indent.is_auto() {
+        if hanging_indent.is_auto() && align.x == FixedAlignment::Start {
             let pod = Region::new(Axes::splat(Abs::inf()), Axes::splat(false));
 
             // We don't have a locator for the numbering here, so we just

crates/typst-library/src/model/heading.rs

@@ -177,12 +177,16 @@ pub struct HeadingElem {
 
     /// The indent all but the first line of a heading should have.
     ///
-    /// The default value of `{auto}` indicates that the subsequent heading
-    /// lines will be indented based on the width of the numbering.
+    /// The default value of `{auto}` uses the width of the numbering as indent
+    /// if the heading is aligned at the [start]($direction.start) of the [text
+    /// direction]($text.dir), and no indent for center and other alignments.
     ///
     /// ```example
     /// #set heading(numbering: "1.")
-    /// #heading[A very, very, very, very, very, very long heading]
+    /// = A very, very, very, very, very, very long heading
+    ///
+    /// #show heading: set align(center)
+    /// == A very long heading\ with center alignment
     /// ```
     #[default(Smart::Auto)]
     pub hanging_indent: Smart<Length>,

tests/suite/model/heading.typ

@@ -101,6 +101,11 @@ comment spans lines
 #set heading(numbering: "1.1.a.")
 = State of the Art
 
+--- heading-hanging-indent-auto-center-align ---
+#set heading(numbering: "1.1.a.")
+#show heading: set align(center)
+= Center aligned
+
 --- heading-hanging-indent-zero ---
 #set heading(numbering: "1.1.a.", hanging-indent: 0pt)
 = State of the Art