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

On Mozilla Firefox when selecting a line in the code block using a triple click, it selects it fine. But if you copy it, it copies both the line you selected and one line above. This is really annoying when copy&pasting specific lines for example in bash scripts or config files. Using triple click in normal text paragraphs works fine.

Reproducible demo

No response

Steps to reproduce

1. Open a docusaurus website containing code blocks with multiple lines, e.g. https://docusaurus.io/docs/next/cli
2. Select a line of code using triple-click
3. Copy it
4. Paste it somewhere

Expected behavior

The copied line is pasted.

Actual behavior

Two lines are pasted: The copied line and one before.

Your environment

• Public source code: https://github.com/oasisprotocol/docs
• Public site URL: https://docs.oasis.io/rofl/app#app-definition
• Docusaurus version used: 3.5.2
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Mozilla Firefox 130.0
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Ubuntu 24.04

Self-service

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