Fix custom browser user-agent failing the flow when canOpenURL: returns NO

CHANGELOG.md

@@ -1,5 +1,6 @@
 # UNRELEASED
 - Improved the state validation error messages to distinguish a missing `state` parameter from a mismatched one. The end session message now notes that a provider's `end_session_endpoint` may point at a legacy or SAML single-logout endpoint, which does not echo `state`. Addresses issue #956.
+- Fixed `OIDExternalUserAgentIOSCustomBrowser` failing the flow with "Unable to open Safari." when `canOpenURL:` returns `NO` for a URL that the system can open, for example an https URL when Safari is restricted and another browser is the default. The flow now fails only if the URL really cannot be opened. Addresses issue #980.
 
 # 3.0.0
 - BREAKING: Updates made to support Xcode 27. ([#972](https://github.com/openid/AppAuth-iOS/pull/972), [#973](https://github.com/openid/AppAuth-iOS/pull/973))

Sources/AppAuth/iOS/OIDExternalUserAgentIOSCustomBrowser.m

@@ -27,6 +27,7 @@
 #import "OIDAuthorizationRequest.h"
 #import "OIDAuthorizationService.h"
 #import "OIDErrorUtilities.h"
+#import "OIDExternalUserAgentSession.h"
 #import "OIDURLQueryComponent.h"
 
 #if !TARGET_OS_MACCATALYST
@@ -153,9 +154,24 @@ - (BOOL)presentExternalUserAgentRequest:(nonnull id<OIDExternalUserAgentRequest>
   // Transforms the request URL and opens it.
   NSURL *requestURL = [request externalUserAgentRequestURL];
   requestURL = _URLTransformation(requestURL);
-  BOOL willOpen = [[UIApplication sharedApplication] canOpenURL:requestURL];
-  [[UIApplication sharedApplication] openURL:requestURL options:@{} completionHandler:nil];
-  return willOpen;
+  if (!requestURL) {
+    return NO;
+  }
+  // -canOpenURL: is not used to decide whether the request was presented: it is subject to
+  // LSApplicationQueriesSchemes and can return NO for a URL that
+  // -openURL:options:completionHandler: opens successfully (e.g. an https URL when Safari is
+  // unavailable and another browser is the default). The flow fails only if the open really fails.
+  [[UIApplication sharedApplication] openURL:requestURL
+                                     options:@{}
+                           completionHandler:^(BOOL success) {
+    if (!success) {
+      NSError *safariError = [OIDErrorUtilities errorWithCode:OIDErrorCodeSafariOpenError
+                                              underlyingError:nil
+                                                  description:@"Unable to open Safari."];
+      [session failExternalUserAgentFlowWithError:safariError];
+    }
+  }];
+  return YES;
 }
 
 - (void)dismissExternalUserAgentAnimated:(BOOL)animated