The helper's key choice is what it avoids doing. Emptying the app root with `app.innerHTML = ""` is the obvious way to clear the page, and according to the writeup it breaks Vue: the vnodes still point at the nodes you deleted, so the next router navigation patches elements that are no longer in the document [13]. Replacing the root element with a comment placeholder and putting the element back afterwards leaves that correspondence intact, which is why `restorePage()` hands back a live app instead of a husk [12][14].
`prepend` is doing layout work too. The page is already offset to make room for the twd sidebar, and anything left in normal flow inherits that offset, so a host div prepended to `body` sits where a user would actually see it [15]. Appending is what `render()` does by default, which is how the component ends up after `#app`, a full viewport below the layout [8], on a page where `screen` also still matches the running app's own elements [9].
Adoption cost, counted off the listing: one support file with two exported functions and two internal ones, and no changes to the application [22]. The cost that matters shows up at runtime instead. jsdom tears the environment down between test files; a browser does not, so renders accumulate and the app has to be back on the page before your flow tests run [17]. That is why `cleanup()` and `restorePage()` both sit in `afterEach` rather than `beforeEach` [16]. Per-test hygiene stops being the runner's job and becomes yours.
For this to transfer, a few things have to be true of your suite. Your component tests need to be Testing Library calls rather than runner-coupled mounts, because the whole move rests on `render()` delegating to `@vue/test-utils` and `@testing-library/dom`, neither of which is bound to jsdom [7]. Your app root needs to be `#app`, which the helper hardcodes as `APP_ROOT_ID` [20]. And you need the dev server in the loop, since twd-js runs the tests inside it and displays them in a sidebar [4]. The post also carries a second example on a view that fetches on mount and posts a form, mocked with `twd.mockReq` [21].
The evidence here comes with caveats. This is one person's report, and that person co-maintains the tool [5]. The demonstration test increments a counter from "Count is 0" to "Count is 1" on click [19], which is the assertion jsdom was never going to get wrong. So the sample proves the plumbing holds, not that it caught a bug. What the real page adds is the ability to assert on size and position at all, which is exactly the region a green jsdom run says nothing about [3].
