Many libraries check for the presence of window to decide if they are running in the browser or on the server. This can pose a problem when trying to run such a library alongside Lit SSR, because the Lit SSR DOM shim defines window, so libraries that would usually be compatible with Node get confused about where they're running and break.

• Example from Sass: [ssr] Lit SSR breaks Sass with polyfill #2393
• Example from Vue router: https://github.com/lit/lit/blob/main/packages/lit-html/src/is-server.ts

We don't really need to define window in our DOM shim, because any supported global API (e.g. customElements.define) that a Lit component library needs to call for SSR can be referenced without the namespace, or using globalThis instead of window.

However, it is somewhat common to see e.g. window.addEventListener in connectedCallback, so removing window would break those cases. Ultimately, I think it's a better trade off to not define window, and just require those occasional lines to switch to globalThis. (We don't actually call connectedCallback or shim addEventListener currently, but we likely will soon).

In the immediate term, to avoid a breaking change, maybe we should make the non-window defining version opt-in. We could export another function from dom-shim.js called e.g. installDomShimWithoutWindow -- and then eventually make this the standard behavior.

(Side note: our implementation of isServer uses node export conditions instead of checking any globals, which I think would be a great direction for other libraries to move in, since it avoids this problem and also avoids any runtime check: https://github.com/lit/lit/blob/main/packages/lit-html/src/is-server.ts)