fix(reconciler): reconcile Flyout for DropDownButton + SplitButton + ToggleSplitButton

CHANGELOG.md

@@ -146,6 +146,27 @@ to land under these conventions; subsequent specs follow this shape.
 
 ### Fixed
 
+- **`DropDownButton` / `SplitButton` / `ToggleSplitButton` `.Flyout` now
+  reconciles across re-renders.** Mount built each button's flyout once
+  and the corresponding Update paths never rebuilt it, so any dynamic
+  flyout content (a `MenuFlyoutContentElement` whose `Items` array is
+  state-derived, a `ContentFlyoutElement` whose subtree depends on
+  state, or a swapped flyout instance entirely) froze at first-mount
+  values. The matching X→null transition (a button whose flyout is
+  removed across renders) also left the stale flyout attached.
+  `UpdateDropDownButton`, `UpdateSplitButton`, and
+  `UpdateToggleSplitButton` now thread the old element +
+  `requestRerender` and call the existing `ApplyFlyoutAttachment`
+  helper, which already knows how to reuse the realized `FlyoutBase`
+  (so an already-open flyout stays open), clear+repopulate
+  `MenuFlyout.Items`, or reconcile content inside a `Flyout.Content`
+  child; an explicit `Flyout = null` branch handles the clear case.
+  Same class of bug as the recently-fixed CommandBar / TeachingTip
+  Content gap (microsoft-ui-reactor#343). Adds `FlyoutReconcileFixtures`
+  (six selftests covering text mutation, count mutation,
+  `ContentFlyoutElement` subtree mutation, and X→null clearing across
+  the three button kinds).
+
 - **ListView / GridView / LazyVStack / LazyHStack now surface incremental
   WinUI deltas for keyed updates.** Previously, any `ItemCount` change
   rebuilt `ItemsSource` from `Enumerable.Range(...)`, which caused WinUI

src/Reactor/Core/Reconciler.Update.cs

@@ -124,12 +124,12 @@ public sealed partial class Reconciler
                 => UpdateRepeatButton(o, n, rb),
             (ToggleButtonElement o, ToggleButtonElement n, WinPrim.ToggleButton tb)
                 => UpdateToggleButton(o, n, tb),
-            (DropDownButtonElement, DropDownButtonElement n, WinUI.DropDownButton ddb)
-                => UpdateDropDownButton(n, ddb),
+            (DropDownButtonElement o, DropDownButtonElement n, WinUI.DropDownButton ddb)
+                => UpdateDropDownButton(o, n, ddb, requestRerender),
             (SplitButtonElement o, SplitButtonElement n, WinUI.SplitButton sb)
-                => UpdateSplitButton(o, n, sb),
+                => UpdateSplitButton(o, n, sb, requestRerender),
             (ToggleSplitButtonElement o, ToggleSplitButtonElement n, WinUI.ToggleSplitButton tsb)
-                => UpdateToggleSplitButton(o, n, tsb),
+                => UpdateToggleSplitButton(o, n, tsb, requestRerender),
             (RichEditBoxElement o, RichEditBoxElement n, WinUI.RichEditBox reb)
                 => UpdateRichEditBox(o, n, reb),
             (TextFieldElement o, TextFieldElement n, TextBox tb)
@@ -736,24 +736,32 @@ private static void RebuildRichTextBlocks(RichTextBlockElement n, WinUI.RichText
         return null;
     }
 
-    private UIElement? UpdateDropDownButton(DropDownButtonElement n, WinUI.DropDownButton ddb)
+    private UIElement? UpdateDropDownButton(DropDownButtonElement o, DropDownButtonElement n, WinUI.DropDownButton ddb, Action requestRerender)
     {
         if (ddb.Content as string != n.Label) ddb.Content = n.Label;
         SetElementTag(ddb, n);
+        if (n.Flyout is not null)
+            ApplyFlyoutAttachment(ddb, o.Flyout, n.Flyout, requestRerender);
+        else if (o.Flyout is not null)
+            ddb.Flyout = null;
         ApplySetters(n.Setters, ddb);
         return null;
     }
 
-    private UIElement? UpdateSplitButton(SplitButtonElement o, SplitButtonElement n, WinUI.SplitButton sb)
+    private UIElement? UpdateSplitButton(SplitButtonElement o, SplitButtonElement n, WinUI.SplitButton sb, Action requestRerender)
     {
         sb.Content = n.Label; SetElementTag(sb, n);
         if (o.OnClick is null && n.OnClick is not null)
             sb.Click += (s, _) => (GetElementTag((UIElement)s!) as SplitButtonElement)?.OnClick?.Invoke();
+        if (n.Flyout is not null)
+            ApplyFlyoutAttachment(sb, o.Flyout, n.Flyout, requestRerender);
+        else if (o.Flyout is not null)
+            sb.Flyout = null;
         ApplySetters(n.Setters, sb);
         return null;
     }
 
-    private UIElement? UpdateToggleSplitButton(ToggleSplitButtonElement o, ToggleSplitButtonElement n, WinUI.ToggleSplitButton tsb)
+    private UIElement? UpdateToggleSplitButton(ToggleSplitButtonElement o, ToggleSplitButtonElement n, WinUI.ToggleSplitButton tsb, Action requestRerender)
     {
         SetElementTag(tsb, n);
         if (o.OnIsCheckedChanged is null && n.OnIsCheckedChanged is not null)
@@ -769,6 +777,10 @@ private static void RebuildRichTextBlocks(RichTextBlockElement n, WinUI.RichText
             ChangeEchoSuppressor.BeginSuppress(tsb);
             tsb.IsChecked = n.IsChecked;
         }
+        if (n.Flyout is not null)
+            ApplyFlyoutAttachment(tsb, o.Flyout, n.Flyout, requestRerender);
+        else if (o.Flyout is not null)
+            tsb.Flyout = null;
         ApplySetters(n.Setters, tsb);
         return null;
     }