feat(call): desktop without WebRTC offers "Open in browser" instead of a dead end

The Linux desktop app runs on WebKitGTK, which ships without WebRTC (2.52
has no RTCPeerConnection; 2.54 disables it outright pending a libwebrtc
backend around 2.56), so calls can't work there. Until now the call button
just disappeared, the call room said "Your browser does not support WebRTC"
with Join disabled, and an incoming call couldn't be answered.

In the desktop app (isTauri) without WebRTC:
- call rooms: "Calls aren't available in the desktop app on Linux yet: its
  web engine has no WebRTC" + an "Open in browser" button;
- incoming-call overlay: the same, with "Answer in browser";
- room header: the call button stays, and opens the room in the browser.

The link is the room in the web app (config.json `webAppUrl`, https only,
new key); the user presses Join there. Deliberately not an auto-join link:
a crafted URL must not be able to join a call and open someone's mic. It
opens through the desktop's new-window handler (web/mail schemes only → the
system browser). Without `webAppUrl` the explanation shows with no button;
browsers without WebRTC keep the old message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/CallEmbedProvider.tsx

@@ -82,6 +82,7 @@ import { useRoomCallPolicy } from '../hooks/useRoomCallPolicy';
 import { useNotificationsQuiet } from '../hooks/useNotificationsQuiet';
 import { CallAvatarAnimation } from '../styles/Animations.css';
 import { webRTCSupported } from '../utils/rtc';
+import { CallsUnavailableMessage } from '../features/call/CallsUnavailable';
 import { zIndices } from '../styles/zIndex';
 
 const PIP_MIN_W = 200;
@@ -231,13 +232,7 @@ function IncomingCall({ dm, info, onIgnore, onAnswer, onReject }: IncomingCallPr
                 </Text>
               )}
               {!webRTCSupported() && (
-                <Text
-                  style={{ margin: 'auto', color: color.Critical.Main }}
-                  size="L400"
-                  align="Center"
-                >
-                  Your browser does not support WebRTC, which is required for calling.
-                </Text>
+                <CallsUnavailableMessage roomId={room.roomId} actionLabel="Answer in browser" />
               )}
               <Box direction="Column" gap="300">
                 {willAnswerWithCamera && (

src/app/features/call/CallView.tsx

@@ -23,6 +23,7 @@ import { CallMemberRenderer } from './CallMemberCard';
 import * as css from './styles.css';
 import { CallControls } from './CallControls';
 import { useLivekitSupport } from '../../hooks/useLivekitSupport';
+import { CallsUnavailableMessage } from './CallsUnavailable';
 import { webRTCSupported } from '../../utils/rtc';
 
 function LivekitServerMissingMessage() {
@@ -33,25 +34,19 @@ function LivekitServerMissingMessage() {
   );
 }
 
-function WebRTCMissingError() {
-  return (
-    <Text style={{ margin: 'auto', color: color.Critical.Main }} size="L400" align="Center">
-      Your browser does not support WebRTC, which is required for calling.
-    </Text>
-  );
-}
-
 function JoinMessage({
+  roomId,
   hasParticipant,
   livekitSupported,
   rtcSupported,
 }: {
+  roomId: string;
   hasParticipant?: boolean;
   livekitSupported?: boolean;
   rtcSupported?: boolean;
 }) {
   if (rtcSupported === false) {
-    return <WebRTCMissingError />;
+    return <CallsUnavailableMessage roomId={roomId} />;
   }
 
   if (livekitSupported === false) {
@@ -144,6 +139,7 @@ function CallPrescreen() {
             )}
             {!inOtherCall && hasPermission && !channelFull && (
               <JoinMessage
+                roomId={room.roomId}
                 hasParticipant={hasParticipant}
                 livekitSupported={livekitSupported}
                 rtcSupported={rtcSupported}

src/app/features/call/CallsUnavailable.tsx

@@ -0,0 +1,76 @@
+import React, { useMemo } from 'react';
+import { Box, Button, Icon, Icons, Text, color } from 'folds';
+import { useClientConfig } from '../../hooks/useClientConfig';
+import { isTauri } from '../../hooks/useTauri';
+import { webRTCSupported } from '../../utils/rtc';
+import { getCallInBrowserUrl, resolveWebAppUrl } from '../../utils/callInBrowser';
+
+/**
+ * [cinny-desktop: Linux calls] The desktop app on Linux runs on WebKitGTK,
+ * which ships without WebRTC (disabled in 2.54; a libwebrtc backend is due
+ * around 2.56). Where calls can't work here, send the user to the room in the
+ * web app instead of a dead end. Only in the desktop app, and only when
+ * config.json sets `webAppUrl`.
+ */
+export const useCallInBrowserUrl = (roomId: string): string | undefined => {
+  const { webAppUrl } = useClientConfig();
+  return useMemo(
+    () =>
+      isTauri() && !webRTCSupported()
+        ? getCallInBrowserUrl(resolveWebAppUrl(webAppUrl), roomId)
+        : undefined,
+    [webAppUrl, roomId],
+  );
+};
+
+/** Opens through the desktop's new-window handler → the system browser. */
+export const openCallInBrowser = (url: string): void => {
+  window.open(url, '_blank', 'noopener,noreferrer');
+};
+
+const desktopUnavailableText = (): string =>
+  typeof navigator !== 'undefined' && /Linux/i.test(navigator.userAgent)
+    ? 'Calls aren’t available in the desktop app on Linux yet: its web engine has no WebRTC.'
+    : 'Calls aren’t available in this desktop app: its web engine has no WebRTC.';
+
+type CallsUnavailableMessageProps = {
+  roomId: string;
+  /** Button label, e.g. "Answer in browser" for an incoming call. */
+  actionLabel?: string;
+};
+
+export function CallsUnavailableMessage({
+  roomId,
+  actionLabel = 'Open in browser',
+}: CallsUnavailableMessageProps) {
+  const url = useCallInBrowserUrl(roomId);
+
+  if (!isTauri()) {
+    return (
+      <Text style={{ margin: 'auto', color: color.Critical.Main }} size="L400" align="Center">
+        Your browser does not support WebRTC, which is required for calling.
+      </Text>
+    );
+  }
+
+  return (
+    <Box direction="Column" alignItems="Center" gap="200" style={{ margin: 'auto' }}>
+      <Text style={{ color: color.Warning.Main }} size="L400" align="Center">
+        {desktopUnavailableText()}
+        {url ? ' You can join from the web app in your browser.' : ''}
+      </Text>
+      {url && (
+        <Button
+          size="300"
+          variant="Secondary"
+          fill="Soft"
+          radii="300"
+          before={<Icon size="100" src={Icons.External} />}
+          onClick={() => openCallInBrowser(url)}
+        >
+          <Text size="B300">{actionLabel}</Text>
+        </Button>
+      )}
+    </Box>
+  );
+}

src/app/features/room/RoomViewHeader.tsx

@@ -80,6 +80,7 @@ import { widgetsPanelAtom } from '../../state/widgetsPanel';
 import { mobileMembersPanelAtom } from '../../state/mobileMembersPanel';
 import { threadsListAtom } from '../../state/threadsList';
 import { usePendingKnocks } from '../../hooks/usePendingKnocks';
+import { openCallInBrowser, useCallInBrowserUrl } from '../call/CallsUnavailable';
 import { bookmarksPanelAtom } from '../../state/bookmarksPanel';
 
 type RoomMenuProps = {
@@ -465,6 +466,36 @@ function CallRulesChip({ room }: { room: Room }) {
   );
 }
 
+/**
+ * [cinny-desktop: Linux calls] Stand-in for CallButton where this app can't
+ * make calls (desktop on WebKitGTK): opens the room in the web app instead.
+ */
+function CallInBrowserButton({ url }: { url: string }) {
+  return (
+    <TooltipProvider
+      position="Bottom"
+      offset={4}
+      tooltip={
+        <Tooltip>
+          <Text>Call in your browser (this desktop app can’t make calls here)</Text>
+        </Tooltip>
+      }
+    >
+      {(triggerRef) => (
+        <IconButton
+          variant="Surface"
+          fill="None"
+          ref={triggerRef}
+          aria-label="Start call in browser"
+          onClick={() => openCallInBrowser(url)}
+        >
+          <Icon size="400" src={Icons.VideoCamera} />
+        </IconButton>
+      )}
+    </TooltipProvider>
+  );
+}
+
 function CallButton() {
   const room = useRoom();
   const direct = useIsDirectRoom();
@@ -555,6 +586,7 @@ export function RoomViewHeader({ callView }: { callView?: boolean }) {
   );
   const livekitSupported = useLivekitSupport();
   const rtcSupported = webRTCSupported();
+  const callInBrowserUrl = useCallInBrowserUrl(room.roomId);
 
   const [menuAnchor, setMenuAnchor] = useState<RectCords>();
   const [pinMenuAnchor, setPinMenuAnchor] = useState<RectCords>();
@@ -782,11 +814,16 @@ export function RoomViewHeader({ callView }: { callView?: boolean }) {
             {screenSize === ScreenSize.Desktop && <CallRulesChip room={room} />}
             {!room.isCallRoom() &&
               livekitSupported &&
-              rtcSupported &&
+              (rtcSupported || callInBrowserUrl) &&
               hasCallPermission &&
               (direct ||
                 (room.getJoinRule() === 'invite' &&
-                  getStateEvents(room, StateEvent.SpaceParent).length === 0)) && <CallButton />}
+                  getStateEvents(room, StateEvent.SpaceParent).length === 0)) &&
+              (rtcSupported ? (
+                <CallButton />
+              ) : (
+                callInBrowserUrl && <CallInBrowserButton url={callInBrowserUrl} />
+              ))}
             {screenSize === ScreenSize.Desktop && (
               <TooltipProvider
                 position="Bottom"

src/app/hooks/useClientConfig.ts

@@ -25,6 +25,12 @@ export type ClientConfig = {
    * Unset: the bundled copy on this origin. Ignored in the desktop app.
    */
   elementCallUrl?: string;
+
+  /**
+   * Absolute https URL of the public web app (e.g. https://chat.lotusguild.org).
+   * The desktop app sets it so it can hand calls it can't make to the browser.
+   */
+  webAppUrl?: string;
 };
 
 const ClientConfigContext = createContext<ClientConfig | null>(null);

src/app/utils/callInBrowser.test.ts

@@ -0,0 +1,32 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { getCallInBrowserUrl, resolveWebAppUrl } from './callInBrowser';
+
+test('resolveWebAppUrl keeps an https origin (+ path) without a trailing slash', () => {
+  assert.equal(resolveWebAppUrl('https://chat.example.org'), 'https://chat.example.org');
+  assert.equal(resolveWebAppUrl('https://chat.example.org/'), 'https://chat.example.org');
+  assert.equal(resolveWebAppUrl('https://example.org/chat/'), 'https://example.org/chat');
+  assert.equal(resolveWebAppUrl('https://chat.example.org/?x=1#y'), 'https://chat.example.org');
+});
+
+test('resolveWebAppUrl rejects anything but absolute https', () => {
+  [
+    undefined,
+    null,
+    1,
+    '',
+    ' ',
+    '/home',
+    'http://chat.example.org',
+    'not a url',
+    ['javascript', 'alert(1)'].join(':'),
+    'data:text/html,x',
+  ].forEach((v) => assert.equal(resolveWebAppUrl(v), undefined, String(v)));
+});
+
+test('getCallInBrowserUrl points at the room in the web app (no auto-join)', () => {
+  const url = getCallInBrowserUrl('https://chat.example.org', '!room:example.org');
+  assert.equal(url, 'https://chat.example.org/home/!room%3Aexample.org');
+  assert.ok(!/join|call=/i.test(url ?? ''));
+  assert.equal(getCallInBrowserUrl(undefined, '!room:example.org'), undefined);
+});

src/app/utils/callInBrowser.ts

@@ -0,0 +1,27 @@
+import { getLotusRoomPermalink } from '../plugins/lotus-permalink';
+
+/**
+ * The public web app's address from config.json `webAppUrl` (the desktop app
+ * sets it; its own origin is a local server). Absolute https only; anything
+ * else is ignored.
+ */
+export const resolveWebAppUrl = (value: unknown): string | undefined => {
+  if (typeof value !== 'string' || value.trim() === '') return undefined;
+  try {
+    const url = new URL(value);
+    if (url.protocol !== 'https:') return undefined;
+    return url.origin + url.pathname.replace(/\/+$/, '');
+  } catch {
+    return undefined;
+  }
+};
+
+/**
+ * Where to send the user to take a call this app can't make: the room in the
+ * web app (they press Join there themselves — never an auto-join link, which
+ * a crafted URL could abuse to open someone's mic).
+ */
+export const getCallInBrowserUrl = (
+  webAppUrl: string | undefined,
+  roomId: string,
+): string | undefined => (webAppUrl ? getLotusRoomPermalink(webAppUrl, roomId) : undefined);