diff --git a/src/App.tsx b/src/App.tsx
index 36afc4c2f..55703f4c0 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -34,6 +34,7 @@ import { MediaDevicesContext } from "./MediaDevicesContext";
 import { getUrlParams, HeaderStyle, useUrlParams } from "./UrlParams";
 import { AppBar } from "./AppBar";
 import { i18n } from "./utils/i18n";
+import { useRootElement } from "./RootElementContext";
 
 const SentryRoute = Sentry.withSentryReactRouterV7Routing(Route);
 
@@ -44,12 +45,11 @@ interface SimpleProviderProps {
 const BackgroundProvider: FC<SimpleProviderProps> = ({ children }) => {
   const { pathname } = useLocation();
   const { background } = useUrlParams();
+  const rootElement = useRootElement();
 
   useEffect(() => {
-    document
-      .getElementsByTagName("body")[0]
-      .setAttribute("data-background", background);
-  }, [pathname, background]);
+    rootElement.setAttribute("data-background", background);
+  }, [pathname, background, rootElement]);
 
   return children;
 };
diff --git a/src/Modal.tsx b/src/Modal.tsx
index e6ffdf450..46316d370 100644
--- a/src/Modal.tsx
+++ b/src/Modal.tsx
@@ -24,6 +24,7 @@ import { Heading, Glass } from "@vector-im/compound-web";
 import styles from "./Modal.module.css";
 import overlayStyles from "./Overlay.module.css";
 import { useMediaQuery } from "./useMediaQuery";
+import { useRootElement } from "./RootElementContext";
 
 export interface Props {
   title: string;
@@ -78,6 +79,7 @@ export const Modal: FC<Props> = ({
   ...rest
 }) => {
   const { t } = useTranslation();
+  const rootElement = useRootElement();
   // Empirically, Chrome on Android can end up not matching (hover: none), but
   // still matching (pointer: coarse) :/
   const touchscreen = useMediaQuery("(hover: none) or (pointer: coarse)");
@@ -100,7 +102,7 @@ export const Modal: FC<Props> = ({
         onOpenChange={onOpenChange}
         dismissible={onDismiss !== undefined}
       >
-        <Drawer.Portal>
+        <Drawer.Portal container={rootElement}>
           <Drawer.Overlay className={classNames(overlayStyles.bg)} />
           <Drawer.Content
             className={classNames(
@@ -155,7 +157,7 @@ export const Modal: FC<Props> = ({
 
     return (
       <DialogRoot open={open} onOpenChange={onOpenChange}>
-        <DialogPortal>
+        <DialogPortal container={rootElement}>
           <DialogOverlay
             className={classNames(overlayStyles.bg, overlayStyles.animate)}
           />
diff --git a/src/RootElementContext.ts b/src/RootElementContext.ts
new file mode 100644
index 000000000..b97b17796
--- /dev/null
+++ b/src/RootElementContext.ts
@@ -0,0 +1,30 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { createContext, use } from "react";
+
+/**
+ * The element that Element Call treats as the root of its own interface.
+ *
+ * Element Call decorates this element with the theme, layout and background
+ * attributes its stylesheets key off, and portals its modals into it. When
+ * Element Call owns the page this is simply the document body; when it is
+ * embedded in a host application it is the container the host mounted it into,
+ * so that Element Call does not reach outside its own subtree.
+ */
+const RootElementContext = createContext<HTMLElement | null>(null);
+
+export const RootElementProvider = RootElementContext.Provider;
+
+/**
+ * The element Element Call should decorate and portal into.
+ *
+ * Defaults to the document body, so that the standalone and widget builds work
+ * without a provider.
+ */
+export const useRootElement = (): HTMLElement =>
+  use(RootElementContext) ?? document.body;
diff --git a/src/Toast.tsx b/src/Toast.tsx
index 83e220bc1..0a2a2f1b8 100644
--- a/src/Toast.tsx
+++ b/src/Toast.tsx
@@ -25,6 +25,7 @@ import { Text } from "@vector-im/compound-web";
 
 import styles from "./Toast.module.css";
 import overlayStyles from "./Overlay.module.css";
+import { useRootElement } from "./RootElementContext";
 
 interface Props {
   /**
@@ -64,6 +65,7 @@ export const Toast: FC<Props> = ({
   Icon,
   modal = true,
 }) => {
+  const rootElement = useRootElement();
   const onOpenChange = useCallback(
     (open: boolean) => {
       if (!open) onDismiss();
@@ -104,7 +106,11 @@ export const Toast: FC<Props> = ({
 
   return (
     <DialogRoot open={open} onOpenChange={onOpenChange} modal={modal}>
-      {modal ? <DialogPortal>{content}</DialogPortal> : content}
+      {modal ? (
+        <DialogPortal container={rootElement}>{content}</DialogPortal>
+      ) : (
+        content
+      )}
     </DialogRoot>
   );
 };
diff --git a/src/room/GroupCallView.tsx b/src/room/GroupCallView.tsx
index fbd589e78..dfeb0866a 100644
--- a/src/room/GroupCallView.tsx
+++ b/src/room/GroupCallView.tsx
@@ -80,6 +80,7 @@ import { useTypedEventEmitter } from "../useEvents";
 import { muteAllAudio$ } from "../state/MuteAllAudioModel.ts";
 import { useAppBarTitle } from "../AppBar.tsx";
 import { useBehavior } from "../useBehavior.ts";
+import { useRootElement } from "../RootElementContext.ts";
 
 /**
  * If there already are this many participants in the call, we automatically mute
@@ -123,6 +124,7 @@ export const GroupCallView: FC<Props> = ({
     null,
   );
   const memberships = useMatrixRTCSessionMemberships(rtcSession);
+  const rootElement = useRootElement();
 
   const muteAllAudio = useBehavior(muteAllAudio$);
   const leaveSoundContext = useLatest(
@@ -150,11 +152,11 @@ export const GroupCallView: FC<Props> = ({
   // viewport sizes smaller than 122px width. (It is actually this exact number: 122px
   // tested on different devices...)
   useEffect(() => {
-    document.body.classList.add("no-scroll-body");
+    rootElement.classList.add("no-scroll-body");
     return (): void => {
-      document.body.classList.remove("no-scroll-body");
+      rootElement.classList.remove("no-scroll-body");
     };
-  }, []);
+  }, [rootElement]);
 
   useEffect(() => {
     window.rtcSession = rtcSession;
diff --git a/src/tile/SpotlightTile.tsx b/src/tile/SpotlightTile.tsx
index 036e044fe..97e1f4a62 100644
--- a/src/tile/SpotlightTile.tsx
+++ b/src/tile/SpotlightTile.tsx
@@ -54,6 +54,7 @@ import { Slider } from "../Slider";
 import { platform } from "../Platform";
 import { type RingingMediaViewModel } from "../state/media/RingingMediaViewModel";
 import { RingingStatus } from "./RingingStatus";
+import { useRootElement } from "../RootElementContext";
 
 interface SpotlightItemBaseProps {
   ref?: Ref<HTMLDivElement>;
@@ -414,6 +415,7 @@ export const SpotlightTile: FC<Props> = ({
   style,
 }) => {
   const { t } = useTranslation();
+  const rootElement = useRootElement();
   const [ourRef, root$] = useObservableRef<HTMLDivElement | null>(null);
   const ref = useMergedRefs(ourRef, theirRef);
   const maximised = useBehavior(vm.maximised$);
@@ -428,24 +430,22 @@ export const SpotlightTile: FC<Props> = ({
   const canGoToNext = visibleIndex !== -1 && visibleIndex < media.length - 1;
 
   const isFullscreen = useCallback((): boolean => {
-    const rootElement = document.body;
     if (rootElement && document.fullscreenElement) return true;
     return false;
-  }, []);
+  }, [rootElement]);
 
   const FullScreenIcon = isFullscreen()
     ? FullScreenMinimiseIcon
     : FullScreenMaximiseIcon;
 
   const onToggleFullscreen = useCallback(() => {
-    const rootElement = document.body;
     if (!rootElement) return;
     if (isFullscreen()) {
       void document?.exitFullscreen();
     } else {
       void rootElement.requestFullscreen();
     }
-  }, [isFullscreen]);
+  }, [isFullscreen, rootElement]);
 
   // To keep track of which item is visible, we need an intersection observer
   // hooked up to the root element and the items. Because the items will run
diff --git a/src/useTheme.ts b/src/useTheme.ts
index e992aee7b..5bac28982 100644
--- a/src/useTheme.ts
+++ b/src/useTheme.ts
@@ -11,12 +11,14 @@ import { type IThemeChangeActionRequest } from "matrix-widget-api";
 
 import { getUrlParams } from "./UrlParams";
 import { widget } from "./widget";
+import { useRootElement } from "./RootElementContext";
 
 export const useTheme = (): void => {
+  const rootElement = useRootElement();
   const [requestedTheme, setRequestedTheme] = useState(
     () => getUrlParams().theme,
   );
-  const previousTheme = useRef<string | null>(document.body.classList.item(0));
+  const previousTheme = useRef<string | null>(rootElement.classList.item(0));
 
   useEffect(() => {
     if (widget) {
@@ -47,15 +49,15 @@ export const useTheme = (): void => {
       : "";
     const themeString = "cpd-theme-" + theme + themeHighContrast;
     if (themeString !== previousTheme.current) {
-      document.body.classList.remove(
+      rootElement.classList.remove(
         "cpd-theme-light",
         "cpd-theme-dark",
         "cpd-theme-light-hc",
         "cpd-theme-dark-hc",
       );
-      document.body.classList.add(themeString);
+      rootElement.classList.add(themeString);
       previousTheme.current = themeString;
     }
-    document.body.classList.remove("no-theme");
-  }, [previousTheme, requestedTheme]);
+    rootElement.classList.remove("no-theme");
+  }, [previousTheme, requestedTheme, rootElement]);
 };