Should fix 4177 navigation to pages with iframe with relative urls are broken

[PH4NTOMiki]

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Fixes #4177
I just did what @Rich-Harris wrote in #4177 (comment) that should fix it.

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[Rich-Harris]

Thanks — can you add a test please?

[PH4NTOMiki]

@Rich-Harris But actually I think this PR may make tests more flaky(and that is the reason we did #4070), is there any way to solve this without doing pushState before root.$set?
What does it even do, I don't really understand?

[Rich-Harris]

Since the DOM update happens in the same turn of the event loop as history.pushState, I don't think this will increase test flakiness — AFAICT Playwright waits until the next tick before resolving waitForNavigation promises. I added a test, and everything is still green (except windows, which is timing out for unrelated reasons), so I think we're good to go