fix(trace) check for pageload count instead of child count (#75516)

We had a test to checking that we skip reparenting cases where server
handlers have multiple direct pageload children, however the actual
condition was checking all children types and not only pageload
children, making the tests pass, but the reparenting to fail on cases
where it should have worked.

static/app/views/performance/newTraceDetails/traceModels/traceTree.tsx

@@ -233,6 +233,21 @@ function isBrowserRequestSpan(value: TraceTree.Span): boolean {
   return value.op === 'browser' && value.description === 'request';
 }
 
+function getPageloadTransactionChildCount(
+  node: TraceTreeNode<TraceTree.NodeValue>
+): number {
+  if (!isTransactionNode(node)) {
+    return 0;
+  }
+  let count = 0;
+  for (const txn of node.value.children) {
+    if (txn && txn['transaction.op'] === 'pageload') {
+      count++;
+    }
+  }
+  return count;
+}
+
 /**
  * Swaps the two nodes in the graph.
  */
@@ -532,8 +547,7 @@ export class TraceTree {
 
     function visit(
       parent: TraceTreeNode<TraceTree.NodeValue | null>,
-      value: TraceTree.Transaction | TraceTree.TraceError,
-      childrenCount: number
+      value: TraceTree.Transaction | TraceTree.TraceError
     ) {
       const node = new TraceTreeNode(parent, value, {
         project_slug:
@@ -597,9 +611,9 @@ export class TraceTree {
       }
 
       if (
-        childrenCount === 1 &&
         isPageloadTransactionNode(node) &&
-        isServerRequestHandlerTransactionNode(parent)
+        isServerRequestHandlerTransactionNode(parent) &&
+        getPageloadTransactionChildCount(parent) === 1
       ) {
         // The swap can occur at a later point when new transactions are fetched,
         // which means we need to invalidate the tree and re-render the UI.
@@ -610,7 +624,7 @@ export class TraceTree {
 
       if (value && 'children' in value) {
         for (const child of value.children) {
-          visit(node, child, value.children.length);
+          visit(node, child);
         }
       }
 
@@ -636,17 +650,17 @@ export class TraceTree {
           typeof orphan.timestamp === 'number' &&
           transaction.start_timestamp <= orphan.timestamp
         ) {
-          visit(traceNode, transaction, transaction.children.length);
+          visit(traceNode, transaction);
           tIdx++;
         } else {
-          visit(traceNode, orphan, 0);
+          visit(traceNode, orphan);
           oIdx++;
         }
       } else if (transaction) {
-        visit(traceNode, transaction, transaction.children.length);
+        visit(traceNode, transaction);
         tIdx++;
       } else if (orphan) {
-        visit(traceNode, orphan, 0);
+        visit(traceNode, orphan);
         oIdx++;
       }
     }