window.opener should not be readonly

[JoshuaKGoldberg]

Reposting microsoft/tslint-microsoft-contrib#293 here:

https://www.jitbit.com/alexblog/256-targetblank---the-most-underestimated-vulnerability-ever/

If you use window.open then you must do this:

let newWnd = window.open();
newWnd.opener = null;

For example, Monaco has to any cast around this:

https://github.com/Microsoft/vscode/blob/d01d178b3b7792d8bc91c03493d5be76b9c4eae8/src/vs/base/browser/dom.ts#L1146

However, opener is marked as readonly. It should not be.

[saschanaz]

AFAIK window.opener has been read-write for years. #141

[Paril]

This does not appear to be an issue in the current state of the code.

[saschanaz]

@github-actions close

[github-actions]

Closing because @saschanaz is one of the code-owners of this repository.