Identify page finished loading content after page was already finished loading

I need write a browser plugin for Chrome where I want to manipulate certain elements. Manipulating the content is not an issue but the page I want to manipulate loads additional content after the page was already done loading.

So my script changes the content but once the page loads the additional content it rebuilds the content and overwrites my changes again.

How can I track those changes or additional loading elements ?

Thanks

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