[PATCH 2/3] fix #4281: ui: fix stale Continue button state on realm switch

[Azharul Haque <haque@azharul.com>]

Form(onChanged: ...) revalidates against whatever fields are
currently mounted at the moment a FormField's own value changes.
For the realm dropdown that runs before the following rebuild
adds/removes the username/password fields for the newly selected
realm's type, so Continue got enabled/disabled based on the
outgoing realm's field set rather than the incoming one. Most
visibly, switching from an OpenID realm to a password realm left
Continue enabled with both fields empty, only failing validation
once actually pressed.

Recompute _submitButtonEnabled explicitly in onDomainChanged instead:
OpenID realms have nothing to validate, so enable it directly; other
realms are revalidated in a post-frame callback once the rebuild has
settled. Do the same after the initial realm auto-selection in
_getAccessDomains(), in case the default realm is an OpenID one.

Signed-off-by: Azharul Haque <haque@azharul.com>
---
 lib/proxmox_login_form.dart | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/lib/proxmox_login_form.dart b/lib/proxmox_login_form.dart
index b73165c..002838c 100644
--- a/lib/proxmox_login_form.dart
+++ b/lib/proxmox_login_form.dart
@@ -396,6 +396,31 @@ class _ProxmoxLoginPageState extends State<ProxmoxLoginPage> {
                                     setState(() {
                                       _selectedDomain = value;
                                     });
+                                    // The Form's onChanged callback validates
+                                    // against whatever fields are mounted at
+                                    // the moment the dropdown's own value
+                                    // changes, which runs before this rebuild
+                                    // adds/removes the username/password
+                                    // fields for the newly selected realm.
+                                    // Recompute once that rebuild has
+                                    // happened so we validate the field set
+                                    // that's actually showing.
+                                    if (value?.isOpenIdRealm == true) {
+                                      setState(() {
+                                        _submitButtonEnabled = true;
+                                      });
+                                    } else {
+                                      WidgetsBinding.instance
+                                          .addPostFrameCallback((_) {
+                                        if (!mounted) return;
+                                        setState(() {
+                                          _submitButtonEnabled = _formKey
+                                                  .currentState
+                                                  ?.validate() ??
+                                              false;
+                                        });
+                                      });
+                                    }
                                   },
                                   onOriginSubmitted: () {
                                     final isValid =
@@ -828,6 +853,9 @@ class _ProxmoxLoginPageState extends State<ProxmoxLoginPage> {
     setState(() {
       _progressModel.inProgress -= 1;
       _selectedDomain = selection;
+      // An OpenID realm has no username/password to fill in, so there's
+      // nothing for the form to validate before Continue is usable.
+      _submitButtonEnabled = selection?.isOpenIdRealm == true;
     });
 
     return response;
-- 
2.50.1 (Apple Git-155)