Cover Element Call as a component with end-to-end tests

The widget tests cannot reach what makes a component different, because the
iframe used to guarantee it: that Element Call stays inside the space it was
given, and that two of them can exist in one page. So the development harness
gets driven by Playwright.

Three tests. Two components in one page holding a real call between two devices
of one account. The settings dialog and the reaction picker staying inside the
container the host gave, asserted by bounding box. And the host bridge
reporting in both directions, including a host-initiated mute coming back as a
report of the new state.

The containment test is the one worth having: both of the escapes found by
running the harness by hand — the settings dialog centred on the window, and
the reaction picker at 82vh landing below the container — would have failed it,
and neither was visible to typechecking, linting or the unit tests.

Users and rooms are created through the Synapse admin and client-server APIs
rather than by driving an interface, and the harness now takes its credentials
from its own query string so that a test can say which account and room to use.
A host reading its own URL is proper; it was Element Call doing so that was the
mistake.

Playwright gains a second web server for the harness on port 3001, a Vite dev
server whether or not the app itself is served from Docker, since the harness is
a development page with nothing to build.

component/dev/Harness.tsx

@@ -37,16 +37,31 @@ const DEFAULT_CREDENTIALS: Credentials = {
   room: "",
 };
 
-/** The last credentials used, so that a reload does not mean typing them again. */
+/**
+ * The credentials to start with: the last ones used, so that a reload does not
+ * mean typing them again, overridden by anything in the query string.
+ *
+ * A host reading its own URL is entirely proper — it was Element Call doing so
+ * that was the mistake. It lets the end-to-end tests, or a shared link, say
+ * which account and room to use.
+ */
 function loadCredentials(): Credentials {
+  let stored: Partial<Credentials> = {};
   try {
-    const stored = localStorage.getItem(CREDENTIALS_KEY);
-    if (stored !== null)
-      return { ...DEFAULT_CREDENTIALS, ...(JSON.parse(stored) as Credentials) };
+    const json = localStorage.getItem(CREDENTIALS_KEY);
+    if (json !== null) stored = JSON.parse(json) as Credentials;
   } catch (e) {
     logger.warn("Could not read the stored harness credentials", e);
   }
-  return DEFAULT_CREDENTIALS;
+
+  const query = new URLSearchParams(location.search);
+  const fromUrl = Object.fromEntries(
+    (["homeserver", "username", "password", "room"] as const)
+      .map((name) => [name, query.get(name)])
+      .filter(([, value]) => value !== null),
+  ) as Partial<Credentials>;
+
+  return { ...DEFAULT_CREDENTIALS, ...stored, ...fromUrl };
 }
 
 interface Session {
@@ -88,7 +103,7 @@ const Pane: FC<{
   );
 
   return (
-    <section className={styles.pane}>
+    <section className={styles.pane} data-testid="call-pane">
       <div className={styles.paneBar}>
         <strong>{session.label}</strong>
         <code>{session.client.getDeviceId()}</code>
@@ -110,7 +125,7 @@ const Pane: FC<{
       </div>
       {/* Resizable, because how Element Call copes with the size it is given is
       one of the things we cannot find out from the standalone app */}
-      <div className={styles.paneCall}>
+      <div className={styles.paneCall} data-testid="call-container">
         {mounted && (
           <ElementCall
             client={session.client}
@@ -281,7 +296,7 @@ export const Harness: FC = (): ReactNode => {
           ))}
         </main>
       </div>
-      <section className={styles.log}>
+      <section className={styles.log} data-testid="bridge-log">
         <h2>Host bridge</h2>
         <ol>
           {entries.map((entry, i) => (