fix: issues on performances using sections

lib/BigListProcessor.js

@@ -205,6 +205,23 @@ export default class BigListProcessor {
       }
     };
 
+    /**
+     * Calculate spacer height.
+     */
+    const getSpacerHeight = () => {
+      let itemsCounter = -1;
+      return items.reduce((totalHeight, item, i) => {
+        if (i !== items.length - 1) {
+          const fullRow = isFullRow(item);
+          itemsCounter = fullRow ? 0 : itemsCounter + 1;
+          if (fullRow || itemsCounter % this.numColumns === 0) {
+            return totalHeight + item.height;
+          }
+        }
+        return totalHeight;
+      }, 0);
+    };
+
     // Header
     const headerHeight = this.getHeaderHeight();
     if (headerHeight > 0) {
@@ -228,9 +245,29 @@ export default class BigListProcessor {
       const sectionHeaderHeight = this.getSectionHeaderHeight(section);
       position = height;
       height += sectionHeaderHeight;
+      if (
+        section > 1 &&
+        items.length > 0 &&
+        items[items.length - 1].type === BigListItemType.SECTION_HEADER
+      ) {
+        // Top Spacer
+        const initialSpacerHeight = getSpacerHeight();
+        const prevSection = items[items.length - 1];
+        items.splice(0, items.length);
+        push(
+          {
+            type: BigListItemType.SPACER,
+            position: 0,
+            height: initialSpacerHeight,
+            section: prevSection.section,
+            index: 0,
+          },
+          prevSection,
+        );
+      }
       pushItem({
         type: BigListItemType.SECTION_HEADER,
-        position: section > 0 ? position - headerHeight : position,
+        position: position,
         height: sectionHeaderHeight,
         section: section,
       });