[labs/observers] Prevent failing in SSR

[a11delavar]

Which package(s) are affected?

Observers (@lit-labs/observers)

Description

All observer controllers crash when used in SSR as their constructors don't count on the fact that the window object is not available in the server environment.

Reproduction

Any component that uses some observer controller will fail in SSR:

@customElement('some-component')
class SomeComponent extends LitElement {
    readonly mutationController = new MutationController(this, {
        config: {
	    subtree: true,
	    characterData: true,
	    childList: true,
        })
}

Workaround

You could just add an inline isServer check and not initialized the controller. Not very ergonomic though!

@customElement('some-component')
class SomeComponent extends LitElement {
-    readonly mutationController = new MutationController(this, {
+    readonly mutationController = isServer ? undefined : new MutationController(this, {
        config: {
	    subtree: true,
	    characterData: true,
	    childList: true,
        })
}

Is this a regression?

No or unsure. This never worked, or I haven't tried before.

Affected versions

All versions

Browser/OS/Node environment

All Node versions