Recognise required_on_save flag being set on model fields

It is clear from the test models and bakerydemo that title is frequently declared with a plain FieldPanel rather than TitleFieldPanel, so we should not rely on the presence of TitleFieldPanel or an explicit required_on_save flag on the FieldPanel to enforce form-level validation on the title.

wagtail/admin/panels/field_panel.py

@@ -25,7 +25,7 @@ class FieldPanel(Panel):
         disable_comments=None,
         permission=None,
         read_only=False,
-        required_on_save=False,
+        required_on_save=None,
         **kwargs,
     ):
         super().__init__(**kwargs)
@@ -55,7 +55,21 @@ class FieldPanel(Panel):
         opts = {
             "fields": [self.field_name],
         }
-        if not self.required_on_save:
+
+        required_on_save = self.required_on_save
+        if required_on_save is None:
+            # If required_on_save is not explicitly set, treat it as false unless:
+            # - it corresponds to a model field with required_on_save=True (such as page title)
+            # - it corresponds to a non-null, non-text-typed model field (in which case a blank value
+            #   is not valid at the database level)
+            try:
+                db_field = self.db_field
+            except FieldDoesNotExist:
+                required_on_save = False
+            else:
+                required_on_save = getattr(db_field, "required_on_save", False)
+
+        if not required_on_save:
             opts["defer_required_on_fields"] = [self.field_name]
 
         if self.widget:

wagtail/admin/panels/title_field_panel.py

@@ -32,7 +32,6 @@ class TitleFieldPanel(FieldPanel):
         **kwargs,
     ):
         kwargs["classname"] = classname
-        kwargs.setdefault("required_on_save", True)
         self.apply_if_live = apply_if_live
         self.placeholder = placeholder
         self.targets = targets

wagtail/models/pages.py

@@ -283,6 +283,7 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
         max_length=255,
         help_text=_("The page title as you'd like it to be seen by the public"),
     )
+    title.required_on_save = True
     # to reflect title of a current draft in the admin UI
     draft_title = models.CharField(max_length=255, editable=False)
     slug = models.SlugField(