perf(mobile): flatten large file trees without copying descendants

apps/mobile/src/features/files/fileTree.test.ts

@@ -84,3 +84,17 @@ describe("mobile file tree helpers", () => {
     expect([...defaultExpandedTreePaths(tree)]).toEqual(["src"]);
   });
 });
+
+it("flattens a wide expanded directory without exceeding argument limits", () => {
+  const children = Array.from({ length: 200_000 }, (_, index) => ({
+    path: `root/file-${index}`,
+    name: `file-${index}`,
+    kind: "file" as const,
+    children: [],
+  }));
+  const nodes = [{ path: "root", name: "root", kind: "directory" as const, children }];
+  const rows = flattenFileTree({ nodes, expanded: new Set(["root"]) });
+  expect(rows).toHaveLength(children.length + 1);
+  expect(rows[0]).toEqual({ node: nodes[0], depth: 0 });
+  expect(rows.at(-1)).toEqual({ node: children.at(-1), depth: 1 });
+});

apps/mobile/src/features/files/fileTree.ts

@@ -110,23 +110,23 @@ function flattenNode(
   const isSearching = searchResultPaths !== null;
   const matches = searchResultPaths?.has(node.path) ?? false;
   let descendantMatches = false;
-  const childOutput: VisibleFileTreeNode[] = [];
+  const start = output.length;
+  output.push({ node, depth });
 
   if (node.kind === "directory" && (expanded.has(node.path) || isSearching)) {
     for (const child of node.children) {
-      if (flattenNode(childOutput, child, depth + 1, expanded, searchResultPaths)) {
+      if (flattenNode(output, child, depth + 1, expanded, searchResultPaths)) {
         descendantMatches = true;
       }
     }
   }
 
   const visible = !isSearching || matches || descendantMatches;
   if (!visible) {
+    output.length = start;
     return false;
   }
 
-  output.push({ node, depth });
-  output.push(...childOutput);
   return matches || descendantMatches;
 }