navigation to pages with iframes with relative URLs are broken after #4070

[rgossiaux]

Describe the bug

• You have a page at "/foo/bar" which loads an iframe whose source is the relative URL "baz" (the absolute URL is thus "/foo/baz")
• You navigate from "/foo" to "/foo/bar"
• The iframe loads with the wrong relative URL--it should load with the relative URL of the page it's on (so "/foo/baz") but instead it loads relative to the original page ("/baz")
• Refresh and everything works; the error is only on navigation.

This is a regression. I bisected the kit version responsible for this and it's 1.0.0-next.285 which looks to correspond to this PR: #4070

Reproduction

You can repro on this branch: rgossiaux/svelte-headlessui#74

• Clone the repo
• npm install
• git checkout upgrade-kit
• npm run dev
• Go to the /docs home page
• Click on Dialog on the sidebar

The iframe will 404. After a refresh it will load.

Logs

No response

System Info

System:
    OS: macOS 12.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 5.59 GB / 64.00 GB
    Shell: 3.3.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 16.8.0 - /var/folders/rn/2kdgb0jj25q2v656j3qc_ywr0000gn/T/fnm_multishells/2969_1644654672256/bin/node
    Yarn: 1.22.17 - /opt/homebrew/bin/yarn
    npm: 7.21.0 - /var/folders/rn/2kdgb0jj25q2v656j3qc_ywr0000gn/T/fnm_multishells/2969_1644654672256/bin/npm
  Browsers:
    Chrome: 98.0.4758.109
    Safari: 15.2
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.4
    @sveltejs/kit: ^1.0.0-next.285 => 1.0.0-next.285
    svelte: ^3.44.0 => 3.44.3

Severity

blocking an upgrade

Additional Information

No response

[Rich-Harris]

I expect we could fix this by doing the pushState immediately before root.$set. It might make the code more convoluted, and we should wait until #4161 is merged before tackling it

[rgossiaux]

Wow, that was fast-- thanks @PH4NTOMiki and @Rich-Harris, you guys are crushing it 😃