Working on new root canvas stuff

Source/Integration/PlayerControllerBase.cpp

@@ -256,6 +256,10 @@ void AlxPlayerControllerBase::BuildInputStack(TArray<UInputComponent*>& InputSta
 
 void AlxPlayerControllerBase::UpdateInputMode()
 {
+	// Drain any deferred ZOrder writes from SetWidgetWindowManagement
+	// before we read the front-most slot.
+	RootCanvas->PropagateZOrderToSlate();
+
 	// Get all the various objects we need to be able to manipulate
 	// the input mode.
 	UGameViewportClient *GameViewportClient = GetWorld()->GetGameViewport();

Source/Integration/RootCanvas.cpp

@@ -54,6 +54,18 @@ int32 UlxRootCanvasPanel::GetMaxZOrder() const
 	return MaxZOrder;
 }
 
+void UlxRootCanvasPanel::PropagateZOrderToSlate()
+{
+	if (!MustPropagateZOrderToSlate) return;
+	MustPropagateZOrderToSlate = false;
+	for (UPanelSlot *PanelSlot : Slots)
+	{
+		UlxRootCanvasSlot *TypedSlot = Cast<UlxRootCanvasSlot>(PanelSlot);
+		check(TypedSlot);
+		TypedSlot->SetZOrder(TypedSlot->GetZOrder());
+	}
+}
+
 TArray<UlxRootCanvasSlot*> UlxRootCanvasPanel::GetSortedUserWidgets()
 {
 	TArray<UlxRootCanvasSlot*> Result;
@@ -95,7 +107,15 @@ void UlxRootCanvasPanel::SetWidgetWindowManagement(class UUserWidget *Widget,
 	Widget->SetDesiredFocusWidget(DesiredFocusWidget);
 	if (BringToFront)
 	{
-		Slot->SetZOrder(Panel->GetMaxZOrder() + 1);
+		// Write the int32 ZOrder UPROPERTY directly and defer the
+		// Slate-side push to PropagateZOrderToSlate. Going through SetZOrder
+		// here is unsafe when called from Event Construct: the FSlot
+		// has been Exposed but its Owner isn't set yet, which fires
+		// an ensure in SConstraintCanvas::FSlot::SetZOrder.
+		PRAGMA_DISABLE_DEPRECATION_WARNINGS
+		Slot->ZOrder = Panel->GetMaxZOrder() + 1;
+		PRAGMA_ENABLE_DEPRECATION_WARNINGS
+		Panel->MustPropagateZOrderToSlate = true;
 	}
 }
 

Source/Integration/RootCanvas.h

@@ -91,6 +91,13 @@ public:
 	// Used as the basis for placing new widgets on top.
 	int32 GetMaxZOrder() const;
 
+	// SetZOrder is buggy: if you call it inside Event Construct,
+	// it crashes.  So we've developed a hack to make it reliable.  
+	// To set the zorder, you store the value in the slot property,
+	// without calling SetZorder.  This routine, which is used later,
+	// propagates that value into the slate widgets.
+	void PropagateZOrderToSlate();
+
 	// This function updates several window-management-related properties
 	// which are stored in the UserWidget and the lxRootCanvasSlot.  Note that
 	// it is perfectly legal to edit these properties by other means: this
@@ -115,6 +122,10 @@ protected:
 
 	// UPanelWidget
 	virtual UClass* GetSlotClass() const override;
+
+public:
+
+	bool MustPropagateZOrderToSlate = false;
 };