Align point and linebreak should not increase math row height (#4306)

crates/typst/src/math/row.rs

@@ -117,11 +117,19 @@ impl MathRun {
     }
 
     pub fn ascent(&self) -> Abs {
-        self.iter().map(MathFragment::ascent).max().unwrap_or_default()
+        self.iter()
+            .filter(|e| affects_row_height(e))
+            .map(|e| e.ascent())
+            .max()
+            .unwrap_or_default()
     }
 
     pub fn descent(&self) -> Abs {
-        self.iter().map(MathFragment::descent).max().unwrap_or_default()
+        self.iter()
+            .filter(|e| affects_row_height(e))
+            .map(|e| e.descent())
+            .max()
+            .unwrap_or_default()
     }
 
     pub fn class(&self) -> MathClass {
@@ -386,3 +394,7 @@ impl MathRunFrameBuilder {
         frame
     }
 }
+
+fn affects_row_height(fragment: &MathFragment) -> bool {
+    !matches!(fragment, MathFragment::Align | MathFragment::Linebreak)
+}

tests/suite/math/equation.typ

@@ -256,6 +256,16 @@ $ q &= sum_k ln A \
   ],
 )
 
+--- issue-4187-alignment-point-affects-row-height ---
+// In this bug, a row of "-" only should have a very small height; but
+// after adding an alignment point "&", the row gains a larger height.
+// We need to test alignment point "&" does not affect a row's height.
+#box($ - - $, fill: silver)
+#box($ - - $, fill: silver) \
+#box($ a \ - - $, fill: silver)
+#box($ &- - \ &a $, fill: silver)
+#box($ &a \ &- - $, fill: silver)
+
 --- issue-numbering-hint ---
 // In this bug, the hint and error messages for an equation
 // being reference mentioned that it was a "heading" and was