Don't number empty regions of broken block equations (#4836)

crates/typst/src/math/equation.rs

@@ -332,8 +332,10 @@ fn layout_equation_block(
         .layout_into_run(elem, styles)?
         .multiline_frame_builder(&ctx, styles);
     let width = full_equation_builder.size.x;
+    let can_break =
+        BlockElem::breakable_in(styles) && full_equation_builder.frames.len() > 1;
 
-    let equation_builders = if BlockElem::breakable_in(styles) {
+    let equation_builders = if can_break {
         let mut rows = full_equation_builder.frames.into_iter().peekable();
         let mut equation_builders = vec![];
         let mut last_first_pos = Point::zero();
@@ -415,9 +417,14 @@ fn layout_equation_block(
     };
 
     // Add equation numbers to each equation region.
+    let region_count = equation_builders.len();
     let frames = equation_builders
         .into_iter()
         .map(|builder| {
+            if builder.frames.is_empty() && region_count > 1 {
+                // Don't number empty regions, but do number empty equations.
+                return builder.build();
+            }
             add_equation_number(
                 builder,
                 number.clone(),

tests/suite/math/equation.typ

@@ -256,6 +256,10 @@ $ q &= sum_k ln A \
   ],
 )
 
+--- math-equation-number-empty ---
+// Test numbering on empty equations.
+#math.equation(numbering: "1", block: true, [])
+
 --- 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.

tests/suite/math/multiline.typ

@@ -133,3 +133,15 @@ $ a &+ b + & c \
 --- issue-1948-math-text-break ---
 // Test text with linebreaks in math.
 $ x := "a\nb\nc\nd\ne" $
+
+
+--- issue-4829-math-pagebreaking-wrong-number ---
+// Test numbering of empty regions of broken equations.
+#set page(height: 5em)
+#set math.equation(numbering: "1")
+#show math.equation: set block(breakable: true)
+
+#rect(height: 1.5em)
+
+$ a + b \
+  a + b $