Complete hatch creation and editing workflow

src/app/properties.rs

@@ -2057,6 +2057,25 @@ pub(super) fn aggregate_sections(
     for sections in all_sections {
         result = merge_sections(&result, &sections);
     }
+    // Unlike ordinary common properties, cumulative area is an aggregate by
+    // definition. Preserve the individual Area row's "varies" state while
+    // summing every selected hatch's actual filled area (holes subtracted).
+    if selected.len() > 1
+        && selected
+            .iter()
+            .all(|(_, entity)| matches!(entity, acadrust::EntityType::Hatch(_)))
+    {
+        let total = selected
+            .iter()
+            .filter_map(|(_, entity)| match entity {
+                acadrust::EntityType::Hatch(hatch) => {
+                    Some(crate::entities::hatch::boundary_area(hatch))
+                }
+                _ => None,
+            })
+            .sum::<f64>();
+        set_row(&mut result, "cumulative_area", format!("{total:.4}"));
+    }
     result
 }