A couple of the Svelte tutorials use alert (example: https://learn.svelte.dev/tutorial/event-modifiers). After the alert is shown once, no other alert will show up afterwards. Not sure what's going on here, maybe a result of the "we will remove alert support from iframes" push that Google did some time ago.
Also, the iframe is reloaded after each alert, because the "do we still have contact to the iframe" polling doesn't receive any updates while the alert is shown. Not sure how this can be worked around in a way that no false positives/negatives occur.