extensions: TreeItem `no-twisty` CSS not always following TreeItemCollapsibleState in 1.107.x (and Insiders)

[shouples]

Does this issue occur when all (other) extensions are disabled?: Yes

• VS Code Version: 1.107.1
• OS Version: macOS 15.7.2

The main issue I'm seeing is that TreeItems created with the same TreeItemCollapsibleState will occasionally get no-twisty added, which doesn't seem to match the following:

vscode/src/vs/workbench/browser/parts/views/treeView.ts

Lines 1497 to 1503 in ae22fa2

	if (node.parent instanceof Root) {
	if (node.collapsibleState === TreeItemCollapsibleState.None) {
	templateData.container.classList.add('no-twisty');
	} else {
	templateData.container.classList.remove('no-twisty');
	}
	}

For example, in this minimal extension (code further down), tree items with the same collapsible state aren't always showing the same alignment (the ones with no-twisty appear slightly to the right).

TreeItemCollapsibleState.Collapsed

• with no-twisty:

• normal:

TreeItemCollapsibleState.None

• with no-twisty:

• normal:

Steps to Reproduce:

Here's a fairly minimal extension that generates a bunch of tree items with random Collapsed/None states (with a refresh button to generate another random batch):

extension.ts

import * as vscode from "vscode";

interface Item {
  name: string;
}

class Provider implements vscode.TreeDataProvider<Item> {
  getChildren(element?: Item): Item[] {
    const children = [];
    if (!element) {
      // root items
      for (let i = 0; i < 50; i++) {
        children.push({ name: `Item ${i + 1}` });
      }
    } else {
      // child items of `element`
      for (let i = 0; i < 10; i++) {
        children.push({ name: `Sub${element.name}.${i + 1}` });
      }
    }
    return children;
  }

  getTreeItem(element: Item): vscode.TreeItem {
    // random chance some can be expanded, some can't
    const collapsibleState =
      Math.random() < 0.5
        ? vscode.TreeItemCollapsibleState.Collapsed
        : vscode.TreeItemCollapsibleState.None;
    const icon = new vscode.TreeItem(element.name, collapsibleState);
    icon.id = element.name;
    icon.iconPath = new vscode.ThemeIcon("file");
    return icon;
  }

  private _onDidChangeTreeData: vscode.EventEmitter<Item | undefined | void> =
    new vscode.EventEmitter<Item | undefined | void>();
  readonly onDidChangeTreeData: vscode.Event<Item | undefined | void> =
    this._onDidChangeTreeData.event;
  refresh(): void {
    this._onDidChangeTreeData.fire();
  }
}

export function activate(context: vscode.ExtensionContext) {
  const provider = new Provider();
  context.subscriptions.push(
    vscode.window.registerTreeDataProvider("vscode-tree-items-view", provider),
  );

  const refreshCommand = vscode.commands.registerCommand("vscode-tree-items-view.refresh", () =>
    provider.refresh(),
  );
  context.subscriptions.push(refreshCommand);
}

export function deactivate() {}

package.json

{
  "name": "vscode-tree-items",
  "displayName": "vscode-tree-items",
  "description": "",
  "version": "0.1.0",
  "engines": {
    "vscode": "^1.101.0"
  },
  "categories": [
    "Other"
  ],
  "activationEvents": [],
  "main": "./out/extension.js",
  "contributes": {
    "commands": [
      {
        "command": "vscode-tree-items-view.refresh",
        "title": "Generate New Tree Items",
        "icon": "$(refresh)"
      }
    ],
    "viewsContainers": {
      "activitybar": [
        {
          "id": "vscode-tree-items",
          "icon": "$(list-tree)",
          "title": "Tree Items"
        }
      ]
    },
    "views": {
      "vscode-tree-items": [
        {
          "id": "vscode-tree-items-view",
          "icon": "$(list-tree)",
          "name": "Tree Items"
        }
      ]
    },
    "menus": {
      "view/title": [
        {
          "command": "vscode-tree-items-view.refresh",
          "when": "true",
          "group": "navigation"
        }
      ]
    }
  },
  "scripts": {
    "vscode:prepublish": "npm run compile",
    "compile": "tsc -p ./",
    "watch": "tsc -watch -p ./",
    "pretest": "npm run compile && npm run lint",
    "lint": "eslint src",
    "test": "vscode-test"
  },
  "devDependencies": {
    "@types/mocha": "^10.0.9",
    "@types/node": "22.x",
    "@types/sinon": "^17.0.4",
    "@types/vscode": "^1.101.0",
    "@typescript-eslint/eslint-plugin": "^8.10.0",
    "@typescript-eslint/parser": "^8.7.0",
    "@vscode/test-cli": "^0.0.10",
    "@vscode/test-electron": "^2.4.1",
    "assert": "^2.1.0",
    "eslint": "^9.13.0",
    "sinon": "^21.0.0",
    "typescript": "^5.6.3"
  }
}

I also see this on Insiders (note the SubItem 22 children):

Version: 1.108.0-insider (Universal)
Commit: 7f08f95ad54782bd242f5536470b330282197333
Date: 2025-12-19T14:09:25.917Z (1 day ago)

This doesn't seem like the same behavior as #279421, but possibly related? (@alexr00? 👀)

[shouples]

Worth noting I do not see this behavior on 1.106.3: