Fix Android WebAuth callback for system browser flows

src/Essentials/src/WebAuthenticator/WebAuthenticator.android.cs

@@ -13,6 +13,8 @@ partial class WebAuthenticatorImplementation : IWebAuthenticator, IPlatformWebAu
 		Uri currentRedirectUri = null;
 		WebAuthenticatorOptions currentOptions = null;
 
+		internal bool AuthenticatingWithCustomTabs { get; private set; } = false;
+
 		public bool OnResumeCallback(Intent intent)
 		{
 			// If we aren't waiting on a task, don't handle the url
@@ -70,7 +72,11 @@ public async Task<WebAuthenticatorResult> AuthenticateAsync(WebAuthenticatorOpti
 			tcsResponse = new TaskCompletionSource<WebAuthenticatorResult>();
 			currentRedirectUri = callbackUrl;
 
-			if (!(await StartCustomTabsActivity(url)))
+			// Try to start with custom tabs if the system supports it and we resolve it
+			AuthenticatingWithCustomTabs = await StartCustomTabsActivity(url);
+
+			// Fall back to using the system browser if necessary
+			if (!AuthenticatingWithCustomTabs)
 			{
 				// Fall back to opening the system-registered browser if necessary
 				var urlOriginalString = url.OriginalString;

src/Essentials/src/WebAuthenticator/WebAuthenticator.shared.cs

@@ -102,6 +102,11 @@ static IPlatformWebAuthenticatorCallback AsPlatformCallback(this IWebAuthenticat
 			return platform;
 		}
 
+#if ANDROID
+		internal static bool IsAuthenticatingWithCustomTabs(this IWebAuthenticator webAuthenticator)
+			=> (webAuthenticator as WebAuthenticatorImplementation)?.AuthenticatingWithCustomTabs ?? false;
+#endif
+
 		/// <summary>
 		/// Begin an authentication flow by navigating to the specified url and waiting for a callback/redirect to the callbackUrl scheme.
 		/// </summary>

src/Essentials/src/WebAuthenticator/WebAuthenticatorCallbackActivity.android.cs

@@ -10,11 +10,21 @@ protected override void OnCreate(Bundle savedInstanceState)
 		{
 			base.OnCreate(savedInstanceState);
 
-			// start the intermediate activity again with flags to close the custom tabs
-			var intent = new Intent(this, typeof(WebAuthenticatorIntermediateActivity));
-			intent.SetData(Intent.Data);
-			intent.AddFlags(ActivityFlags.ClearTop | ActivityFlags.SingleTop);
-			StartActivity(intent);
+			// Check how we launched the flow initially
+			if (WebAuthenticator.Default.IsAuthenticatingWithCustomTabs())
+			{
+				// start the intermediate activity again with flags to close the custom tabs
+				var intent = new Intent(this, typeof(WebAuthenticatorIntermediateActivity));
+				intent.SetData(Intent.Data);
+				intent.AddFlags(ActivityFlags.ClearTop | ActivityFlags.SingleTop);
+				StartActivity(intent);
+			}
+			else
+			{
+				// No intermediate activity if we returned from a system browser
+				// intent since there's no custom tab instance to clean up
+				WebAuthenticator.Default.OnResume(Intent);
+			}
 
 			// finish this activity
 			Finish();