dialog [nfc]: Disable barrierDismissible on custom onDismiss

Based on the reasoning at the one place we're using either of
these options so far, it seems like we'll always want them
coupled this way.  (At least until some other reason comes along,
and in that case we can revise this logic again.)

Author: gnprice

lib/widgets/dialog.dart

@@ -19,19 +19,22 @@ Future<void> showErrorDialog({
   required BuildContext context,
   required String title,
   String? message,
-  bool barrierDismissible = true,
-  VoidCallback? onContinue,
+  VoidCallback? onDismiss,
 }) {
   final zulipLocalizations = ZulipLocalizations.of(context);
   return showDialog(
     context: context,
-    barrierDismissible: barrierDismissible,
+    // `showDialog` doesn't take an `onDismiss`, so dismissing via the barrier
+    // always causes the default dismiss behavior of popping just this route.
+    // When we want a non-default `onDismiss`, disable that.
+    // TODO(upstream): add onDismiss to showDialog, passing through to [ModalBarrier.onDismiss]
+    barrierDismissible: onDismiss == null,
     builder: (BuildContext context) => AlertDialog(
       title: Text(title),
       content: message != null ? SingleChildScrollView(child: Text(message)) : null,
       actions: [
         TextButton(
-          onPressed: onContinue ?? () => Navigator.pop(context),
+          onPressed: onDismiss ?? () => Navigator.pop(context),
           child: _dialogActionText(zulipLocalizations.errorDialogContinue)),
       ]));
 }

lib/widgets/lightbox.dart

@@ -400,12 +400,7 @@ class _VideoLightboxPageState extends State<VideoLightboxPage> with PerAccountSt
           context: context,
           title: zulipLocalizations.errorDialogTitle,
           message: zulipLocalizations.errorVideoPlayerFailed,
-          // To avoid showing the disabled video lightbox for the unnsupported
-          // video, we make sure user doesn't reach there by dismissing the dialog
-          // by clicking around it, user must press the 'OK' button, which will
-          // take user back to content message list.
-          barrierDismissible: false,
-          onContinue: () {
+          onDismiss: () {
             Navigator.pop(context); // Pops the dialog
             Navigator.pop(context); // Pops the lightbox
           });