Supply Element Call's parameters through a context

Element Call's parameters come from its URL, which works while it owns
the page but leaves an embedder with nowhere to put them. Add a context
so they can be provided directly, falling back to parsing
window.location when no provider is present.

This also decouples the thirteen consumers from react-router: useUrlParams
called useLocation, so each of them required a router ancestor, which the
embedded build will not have. The standalone and widget builds keep their
URL-derived behaviour via useUrlParamsFromLocation, provided in App.

No functional change.

src/App.tsx

@@ -31,7 +31,13 @@ import { useTheme } from "./useTheme";
 import { ProcessorProvider } from "./livekit/TrackProcessorContext";
 import { type AppViewModel } from "./state/AppViewModel";
 import { MediaDevicesContext } from "./MediaDevicesContext";
-import { getUrlParams, HeaderStyle, useUrlParams } from "./UrlParams";
+import {
+  getUrlParams,
+  HeaderStyle,
+  UrlParamsProvider,
+  useUrlParams,
+  useUrlParamsFromLocation,
+} from "./UrlParams";
 import { AppBar } from "./AppBar";
 import { i18n } from "./utils/i18n";
 import { useRootElement } from "./RootElementContext";
@@ -42,6 +48,16 @@ interface SimpleProviderProps {
   children: JSX.Element;
 }
 
+/**
+ * Supplies the URL-derived params to the rest of the app. Only the standalone
+ * and widget builds own the URL, so this lives here in the app shell rather
+ * than alongside the context itself.
+ */
+const LocationUrlParamsProvider: FC<SimpleProviderProps> = ({ children }) => {
+  const urlParams = useUrlParamsFromLocation();
+  return <UrlParamsProvider value={urlParams}>{children}</UrlParamsProvider>;
+};
+
 const BackgroundProvider: FC<SimpleProviderProps> = ({ children }) => {
   const { pathname } = useLocation();
   const { background } = useUrlParams();
@@ -102,6 +118,7 @@ export const App: FC<Props> = ({ vm }) => {
   return (
     <I18nextProvider i18n={i18n}>
       <BrowserRouter>
+        <LocationUrlParamsProvider>
           <BackgroundProvider>
             <ThemeProvider>
               <TooltipProvider>
@@ -115,6 +132,7 @@ export const App: FC<Props> = ({ vm }) => {
               </TooltipProvider>
             </ThemeProvider>
           </BackgroundProvider>
+        </LocationUrlParamsProvider>
       </BrowserRouter>
     </I18nextProvider>
   );

src/UrlParams.ts

@@ -6,7 +6,7 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { useMemo } from "react";
+import { createContext, use, useMemo } from "react";
 import { useLocation } from "react-router-dom";
 import { logger } from "matrix-js-sdk/lib/logger";
 import {
@@ -519,11 +519,36 @@ export const computeUrlParams = (search = "", hash = ""): UrlParams => {
   };
 };
 
+const UrlParamsContext = createContext<UrlParams | null>(null);
+
 /**
- * Hook to simplify use of getUrlParams.
+ * Supplies the parameters Element Call should run with.
- * @returns The app parameters for the current URL
+ *
+ * The standalone and widget builds derive these from the URL, but an embedder
+ * has no URL of its own to put them in, so it provides them directly instead.
+ *
+ * TODO: `UrlParams` is no longer an accurate name now that these need not come
+ * from a URL. Renaming it touches every consumer, so it is left until the rest
+ * of the de-globalisation work has settled.
  */
-export const useUrlParams = (): UrlParams => {
+export const UrlParamsProvider = UrlParamsContext.Provider;
+
+/**
+ * The parameters Element Call is running with.
+ *
+ * Falls back to parsing `window.location` when no provider is present, so that
+ * tests and stories keep working without one.
+ */
+export const useUrlParams = (): UrlParams =>
+  use(UrlParamsContext) ?? getUrlParams();
+
+/**
+ * Derives {@link UrlParams} from the current router location.
+ *
+ * Only meaningful when Element Call owns the URL; embedders provide the params
+ * directly through {@link UrlParamsProvider}.
+ */
+export const useUrlParamsFromLocation = (): UrlParams => {
   const { search, hash } = useLocation();
   return useMemo(() => getUrlParams(search, hash), [search, hash]);
 };