Dialog gets incorrect data-has-nested attribute when mounting with open={true} in Strict Mode

[Alexandre-GL]

🐛 Bug report

When rendering a Dialog with open={true} on mount in React Strict Mode, the dialog content element gets incorrect data-has-nested="dialog" and data-nested attributes, as if a nested dialog were open — even though only a single dialog exists in the DOM.

💥 Steps to reproduce

1. Render a <Dialog.Root open> that mounts with open={true} (controlled, not triggered by user interaction)
2. Run in React development mode (Strict Mode enabled)
3. Inspect the [data-scope="dialog"][data-part="content"] element

Observed:

• data-has-nested="dialog" is set on the only dialog in the DOM
• --layer-index: 1 instead of 0
• --nested-layer-count: 1 instead of 0

In production mode (Strict Mode disabled), the bug does not occur.

💻 Link to reproduction

https://codesandbox.io/p/devbox/github/Alexandre-GL/zag-layerstack-repro/main

GitHub repo: https://github.com/Alexandre-GL/zag-layerstack-repro

git clone <repo>
npm install
npm run dev
# Open http://localhost:5173, click "Mount always-open dialog"

🧐 Expected behavior

A single open dialog should have:

• --layer-index: 0
• --nested-layer-count: 0
• No data-has-nested attribute

🧭 Possible Solution

It seems like layerStack ends up with two entries for the same DOM node. This is likely caused by React Strict Mode's double-invocation of effects, where the cleanup doesn't fully undo the first layerStack.add() call before the second one runs.

A possible fix would be to add a deduplication guard in layerStack.add():

// packages/utilities/dismissable/src/layer-stack.ts
add(layer: Layer) {
  const existingIndex = this.indexOf(layer.node);
  if (existingIndex !== -1) {
    this.layers.splice(existingIndex, 1);
  }
  this.layers.push(layer);
  this.syncLayers();
},

This way, if the same DOM node is added again, the previous entry is removed first — making add() idempotent.

We are currently working around this with a pnpm patch on @zag-js/dismissable using the deduplication guard above. Happy to open a PR if that approach looks right.

🌍 System information

Software	Version
Zag Version	1.39.1
Browser	Chrome 137
Operating System	Linux

Tested via @ark-ui/react@5.36.0 which depends on @zag-js/*@1.39.1.