Strip trailing line breaks in math equations (typst#750)

Author: SUPERCILEX

library/src/math/row.rs

@@ -127,11 +127,15 @@ impl MathRow {
                 TIGHT_LEADING.scaled(ctx)
             };
 
-            let rows: Vec<_> = fragments
+            let mut rows: Vec<_> = fragments
                 .split(|frag| matches!(frag, MathFragment::Linebreak))
                 .map(|slice| Self(slice.to_vec()))
                 .collect();
 
+            if matches!(rows.last(), Some(row) if row.0.is_empty()) {
+                rows.pop();
+            }
+
             let width = rows.iter().map(|row| row.width()).max().unwrap_or_default();
             let points = alignments(&rows);
             let mut frame = Frame::new(Size::zero());

tests/ref/math/multiline.png

@@ -33,3 +33,24 @@ $ "abc" &= c \
 ---
 // Test multiline subscript.
 $ sum_(n in NN \ n <= 5) n = (5(5+1))/2 = 15 $
+
+---
+// Test no trailing line break.
+$
+"abc" &= c
+$
+No trailing line break.
+
+---
+// Test single trailing line break.
+$
+"abc" &= c \
+$
+One trailing line break.
+
+---
+// Test multiple trailing line breaks.
+$
+"abc" &= c \ \ \
+$
+Multiple trailing line breaks.