[Bug] [Safari] Extension conflict on websites with Shopify checkout #15994

Prerequisites

• I searched for any existing report about this bug to avoid opening a duplicate.
• I can reproduce this bug in a new, unmodified web browser profile with Dark Reader installed as the only extension.
• I understand I need to use the Broken Website Report template if this bug I am reporting occurs on a single website.

Bug Description

DarkReader can conflict with other Safari extensions, causing the web page to become unresponsive, on sites with Shopify checkout. This affects my own Safari extension StopTheMadness Pro: https://apps.apple.com/app/stopthemadness-pro/id6471380298

I've created and attached a reduced test case Safari extension to reproduce the issue. The extension creates a new CSSStyleSheet and adds it to the adoptedStyleSheets of the Shadow DOM. It appears that the DarkReader observer for adopted style sheets is running in an infinite loop.

The Shadow DOM in this case is actually the payment buttons, for example, the PayPal button.

By the way, I lied in prerequisites, because I had to: "I can reproduce this bug in a new, unmodified web browser profile with Dark Reader installed as the only extension." Of course since this is an extension conflict, I had to enable a second extension.

Website Address

https://kkandjay.com/

Steps To Reproduce

1. Build and enable the attached Safari extension DarkReaderBug
2. Open https://kkandjay.com/
3. Add an item to the cart
4. Checkout
5. Start entering data in the checkout form

DarkReaderBug.zip

Expected Behavior

The web page remains responsive.

Actual Behavior

The web page becomes unresponsive. The Safari Web Content process is using ~100% CPU.

Screenshots

No response

Operating System

macOS 15.7.9

Web Browser name and version

Safari 26.6.1

Dark Reader version

2.1.10

Additional Context

No response