fix(auth): SSO confirmation works in the desktop app; allow switching to SSO

Reported by a desktop user: signing out other sessions with SSO looped.
Server logs: the SSO step completed every time, but each delete_devices
retry came without the session (a fresh 401 challenge), 4 attempts, none
succeeded. The app is the Windows desktop build (WebView2) and the SSO
pages opened in the system browser.

- SSOStage only offered "Continue" once window.open returned a window. The
  desktop app opens the page in the system browser (no window back), and a
  popup can lose its opener, so "authDone" never arrives and the only way
  on was starting over. Now "Continue" (plus "Open the sign-in page again")
  appears as soon as the page was opened; authDone still auto-continues.
- ActionUIA: when the server offers password and SSO, the first flow was
  forced (often password, which an SSO-only account may not know). Offer
  "Use SSO instead" / "Use your password instead".

e2e: desktop (window.open returns null) completes with the session; with
password offered first, SSO is one click away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/ActionUIA.tsx

@@ -1,4 +1,5 @@
-import React, { ReactNode } from 'react';
+import React, { ReactNode, useState } from 'react';
+import { Box, Button, Text } from 'folds';
 import { AuthDict, AuthType, IAuthData, UIAFlow } from 'matrix-js-sdk';
 import { getUIAFlowForStages } from '../utils/matrix-uia';
 import { useSupportedUIAFlows, useUIACompleted, useUIAFlow } from '../hooks/useUIAFlows';
@@ -23,7 +24,18 @@ type ActionUIAProps = {
 export function ActionUIA({ authData, ongoingFlow, action, onCancel }: ActionUIAProps) {
   const mx = useMatrixClient();
   const completed = useUIACompleted(authData);
-  const { getStageToComplete } = useUIAFlow(authData, ongoingFlow);
+  // The server may offer several ways to confirm (e.g. password or SSO). Start
+  // with the given one but let the user switch: an account that signs in with
+  // SSO may have no password it knows.
+  const supportedFlows = useSupportedUIAFlows(authData.flows ?? [], SUPPORTED_IN_APP_UIA_STAGES);
+  const [flow, setFlow] = useState<UIAFlow>(ongoingFlow);
+  const { getStageToComplete } = useUIAFlow(authData, flow);
+  const other = supportedFlows.find((f) => f.stages.join() !== flow.stages.join());
+  const otherLabel = other?.stages.includes(AuthType.Sso)
+    ? 'Use SSO instead'
+    : other?.stages.includes(AuthType.Password)
+      ? 'Use your password instead'
+      : undefined;
 
   const stageToComplete = getStageToComplete();
 
@@ -31,7 +43,7 @@ export function ActionUIA({ authData, ongoingFlow, action, onCancel }: ActionUIA
   return (
     <UIAFlowOverlay
       currentStep={completed.length + 1}
-      stepCount={ongoingFlow.stages.length}
+      stepCount={flow.stages.length}
       onCancel={onCancel}
     >
       {stageToComplete.type === AuthType.Password && (
@@ -50,6 +62,21 @@ export function ActionUIA({ authData, ongoingFlow, action, onCancel }: ActionUIA
           submitAuthDict={action}
         />
       )}
+      {other && otherLabel && completed.length === 0 && (
+        <Box justifyContent="Center" style={{ marginTop: 8 }}>
+          <Button
+            size="300"
+            variant="Secondary"
+            fill="Soft"
+            radii="300"
+            onClick={() => setFlow(other)}
+          >
+            <Text as="span" size="B300">
+              {otherLabel}
+            </Text>
+          </Button>
+        </Box>
+      )}
     </UIAFlowOverlay>
   );
 }

src/app/components/uia-stages/SSOStage.tsx

@@ -12,6 +12,12 @@ export function SSOStage({
 }) {
   const { errorCode, error, session } = stageData;
   const [ssoWindow, setSSOWindow] = useState<Window>();
+  // Whether the SSO page was opened. Not the same as having a window handle:
+  // the desktop app opens it in the system browser and gets none back, and a
+  // popup can lose its link to us (cross-origin opener policy on the identity
+  // provider). In both cases "authDone" never arrives, so "Continue" must still
+  // be offered once the page was opened, or the only way on is starting over.
+  const [started, setStarted] = useState(false);
 
   const handleSubmit = useCallback(() => {
     submitAuthDict({
@@ -22,6 +28,7 @@ export function SSOStage({
   const handleContinue = () => {
     const w = window.open(ssoRedirectURL, '_blank');
     setSSOWindow(w ?? undefined);
+    setStarted(true);
   };
 
   useEffect(() => {
@@ -74,7 +81,9 @@ export function SSOStage({
         gap="400"
       >
         <Text size="T200">
-          To perform this action you need to authenticate yourself by SSO login.
+          {started
+            ? 'Finish signing in on the page that opened, then come back here and select Continue.'
+            : 'To do this, confirm it’s you by signing in with SSO.'}
         </Text>
         {errorCode && (
           <Box alignItems="Center" gap="100" style={{ color: color.Critical.Main }}>
@@ -85,12 +94,19 @@ export function SSOStage({
           </Box>
         )}
 
-        {ssoWindow ? (
-          <Button variant="Primary" onClick={handleSubmit}>
-            <Text as="span" size="B400">
-              Continue
-            </Text>
-          </Button>
+        {started ? (
+          <Box direction="Column" gap="200">
+            <Button variant="Primary" onClick={handleSubmit}>
+              <Text as="span" size="B400">
+                Continue
+              </Text>
+            </Button>
+            <Button variant="Secondary" fill="None" onClick={handleContinue}>
+              <Text as="span" size="B400">
+                Open the sign-in page again
+              </Text>
+            </Button>
+          </Box>
         ) : (
           <Button variant="Primary" onClick={handleContinue}>
             <Text as="span" size="B400">