Reference

Your component runs in a sandbox

Pasted code is transpiled in the browser and rendered inside an iframe that allows scripts and nothing else: no same-origin access and no network. Everything that comes back out is treated as untrusted.

Nothing you paste runs in the app: The app never runs your code, and it can’t read your component’s DOM directly either. The two sides talk over a versioned message protocol that checks every message’s shape and source.
Picked parts are found by position: A part you pick inside a pasted component is identified by its place in the tree. Repeated class names and no ids are normal in real markup, so a selector could match the wrong one.
A pick follows its part when the code changes: Editing the source can move a part to a new position. Each pick also carries a fingerprint, checked after every render, so a part that moved is followed instead of swapped for whatever now sits there.
Everything from the frame is limited: Paths, labels and fingerprints are all size-limited before they can reach a saved document. Code in the frame could send fake replies, so every message is checked.