Fix equation resizing when adding the equation number (#4179)

crates/typst/src/math/equation.rs

@@ -466,25 +466,33 @@ fn resize_equation(
     if matches!(number_align.y, FixedAlignment::Center if is_multiline) {
         // In this case, the center lines (not baselines) of the number frame
         // and the equation frame shall be aligned.
-        let height = equation.height().max(number.height());
         return equation.resize(
-            Size::new(width, height),
+            Size::new(width, equation.height().max(number.height())),
             Axes::<FixedAlignment>::new(equation_align, FixedAlignment::Center),
         );
     }
 
     let excess_above = Abs::zero().max({
-        let (.., baseline) = first;
-        number.baseline() - baseline
+        if !is_multiline || matches!(number_align.y, FixedAlignment::Start) {
+            let (.., baseline) = first;
+            number.baseline() - baseline
+        } else {
+            Abs::zero()
+        }
     });
     let excess_below = Abs::zero().max({
-        let (size, .., baseline) = last;
-        (number.height() - number.baseline()) - (size.y - baseline)
+        if !is_multiline || matches!(number_align.y, FixedAlignment::End) {
+            let (size, .., baseline) = last;
+            (number.height() - number.baseline()) - (size.y - baseline)
+        } else {
+            Abs::zero()
+        }
     });
-    let height = equation.height() + excess_above + excess_below;
 
+    // The vertical expansion is asymmetric on the top and bottom edges, so we
+    // first align at the top then translate the content downward later.
     let resizing_offset = equation.resize(
-        Size::new(width, height),
+        Size::new(width, equation.height() + excess_above + excess_below),
         Axes::<FixedAlignment>::new(equation_align, FixedAlignment::Start),
     );
     equation.translate(Point::with_y(excess_above));

tests/suite/math/equation.typ

@@ -194,22 +194,67 @@ $ q &= sum_k ln A \
 // numbering's layout box. Note we use pattern "1" here, not "(1)", since
 // the parenthesis exceeds the numbering's layout box, due to the default
 // settings of top-edge and bottom-edge of the TextElem that laid it out.
+#let equations = [
+  #box($ - - - $, fill: silver)
+  #box(
+  $ - - - \
+    a = b $,
+  fill: silver)
+  #box(
+  $ a = b \
+    - - - $,
+  fill: silver)
+]
+
 #set math.equation(numbering: "1", number-align: top)
-#box(
-$ - &- - \
-  a &= b $,
-fill: silver)
+#equations
 
 #set math.equation(numbering: "1", number-align: horizon)
-#box(
-$ - - - $,
-fill: silver)
+#equations
 
 #set math.equation(numbering: "1", number-align: bottom)
-#box(
-$ a &= b \
-  - &- - $,
-fill: silver)
+#equations
+
+--- math-equation-number-align-multiline-no-expand ---
+// Tests that if the numbering's layout box doesn't vertically exceed the
+// box of the equation frame's boundary, the latter's frame size remains.
+// So, in the grid below, frames in each row should have the same height.
+#set math.equation(numbering: "1")
+#grid(
+  columns: 4 * (1fr,),
+  column-gutter: 3 * (2pt,),
+  row-gutter: 2pt,
+  align: horizon,
+  [
+    #set math.equation(number-align: horizon)
+    #box($ - - \ a \ sum $, fill: silver)
+  ],
+  [
+    #set math.equation(number-align: bottom)
+    #box($ - - \ a \ sum $, fill: silver)
+  ],
+  [
+    #set math.equation(number-align: horizon)
+    #box($ sum \ a \ - - $, fill: silver)
+  ],
+  [
+    #set math.equation(number-align: top)
+    #box($ sum \ a \ - - $, fill: silver)
+  ],
+
+  [
+    #set math.equation(number-align: horizon)
+    #box($ - - $, fill: silver)
+  ],
+  [
+    #set math.equation(number-align: top)
+    #box($ - - $, fill: silver)
+  ],
+  [
+    #set math.equation(number-align: bottom)
+    #box($ - - $, fill: silver)
+  ],
+)
 
 --- issue-numbering-hint ---
 // In this bug, the hint and error messages for an equation