Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Bug Report

Docusaurus version: 3.9.2
Node version: v20.19.5
Browser: chrome 145.0.7632.116

Problem

After upgrading from 3.8.x to 3.9.x, the code block copy button throws an error when the site is accessed via a non-HTTPS local network address (e.g.
http://192.168.x.x:3000).

TypeError: Cannot read properties of undefined (reading 'writeText')
at CopyButton/index.js

Root Cause

PR #11422 replaced copy-text-to-clipboard with the native navigator.clipboard.writeText() API. However, navigator.clipboard is undefined in non-HTTPS contexts
(except localhost). The old library had a <textarea> fallback that handled this case.

Reproducible demo

No response

Steps to reproduce

1. Run npm run start -- --host 0.0.0.0
2. Access the site from another device on the same LAN via http://192.168.x.x:3000
3. Click the copy button on any code block

Expected behavior

Code is copied to clipboard successfully.

Actual behavior

The copy button throws an error and nothing is copied:

TypeError: Cannot read properties of undefined (reading 'writeText')
at CopyButton/index.js

Your environment

• Public source code: N/A
• Public site URL: N/A
• Docusaurus version used: 3.9.2
• Environment name and version: Node.js v20.19.5, Chrome 145.0.7632.116
• Operating system and version: Ubuntu 20.04 LTS

Self-service

• I'd be willing to fix this bug myself.