Fix some issues with new root canvas stuff

Source/Integration/RootCanvas.h

@@ -2,13 +2,16 @@
 //
 // RootCanvas.h
 //
-// UlxRootCanvasPanel is a UCanvasPanel subclass whose
-// slots (UlxRootCanvasSlot) carry input-mode configuration
-// in addition to layout. The PlayerController scans these
-// slots, sorted by ZOrder, to arbitrate pointer visibility,
-// capture, focus, and input-component blocking for
-// top-level widgets. ZOrder therefore serves double duty:
-// it determines draw order AND input priority.
+// Luprex provides a "window management system" for root
+// widgets. In this system, all top-level widgets have to go
+// into the root canvas (instead of the viewport).  The
+// window management system monitors the widgets within the
+// root canvas and continuously updates the z-orders, the
+// pointer visibility, the mouse capture mode, the keyboard
+// focus, and enhanced input event routing based on hints
+// and directives given by the widgets.
+//
+// To learn more, read Docs/Luprex-Window-Management.md
 //
 ////////////////////////////////////////////////////////////
 
@@ -29,11 +32,6 @@ class UWidget;
 //
 // UlxRootCanvasSlot
 //
-// Luprex provides a "window management system" for root widgets.
-// This system is documented in Docs/Keyboard-Focus-and-Input-Modes.md
-// The Root Canvas Slot is how widgets ask the window management system
-// to engage certain behaviors.
-//
 ////////////////////////////////////////////////////////////
 UCLASS()
 class INTEGRATION_API UlxRootCanvasSlot : public UCanvasPanelSlot
@@ -60,32 +58,24 @@ public:
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Luprex|Input Mode")
 	bool EnableEnhancedInput = true;
 
-	// Knowing whether the slot is under construction helps us to work
-	// around some bugs in Unreal.  See below.
-	bool SlotUnderConstruction = true;
-	
-	// Reliable version of SetZOrder.  There is a bug in the normal version of
-	// SetZOrder: it crashes if you use it during OnConstruct.  We have a
-	// workaround, but it requires using a different getter and setter.
-	UFUNCTION(BlueprintCallable, Category = "Luprex|Input Mode")
-	void SetZOrderReliable(int32 Order);
+	// Sequence number assigned when this slot was last brought to front.
+	int32 BringToFrontCount = 0;
 
-	// Reliable version of GetZOrder.  There is a bug in the normal version of
-	// SetZOrder: it crashes if you use it during OnConstruct.  We have a
-	// workaround, but it requires using a different getter and setter.
-	UFUNCTION(BlueprintCallable, Category = "Luprex|Input Mode")
-	int32 GetZOrderReliable();
+	// Widget Z Ordering: widgets that show the pointer go on top of windows
+	// that don't, and within a group, widgets that have greater BringToFrontCount
+	// go on top.  Sorting an array with this puts the top windows at the end
+	// of the array.
+	bool operator<(const UlxRootCanvasSlot &Other) const
+	{
+		if (ShowPointer != Other.ShowPointer) return !ShowPointer;
+		return BringToFrontCount < Other.BringToFrontCount;
+	}
 };
 
 ////////////////////////////////////////////////////////////
 //
 // UlxRootCanvasPanel
 //
-// A UCanvasPanel that uses UlxRootCanvasSlot for its
-// children instead of the plain UCanvasPanelSlot. Layout
-// behavior is identical to UCanvasPanel; only the slot
-// type differs.
-//
 ////////////////////////////////////////////////////////////
 UCLASS()
 class INTEGRATION_API UlxRootCanvasPanel : public UCanvasPanel
@@ -94,18 +84,18 @@ class INTEGRATION_API UlxRootCanvasPanel : public UCanvasPanel
 
 public:
 
-	// Convenience wrapper around AddChild that returns the
-	// derived slot type, so callers don't have to cast.
+	// Add a child to the canvas.  Also brings the child to the front
+	// for the first time, ensuring that every widget has a unique
+	// BringToFront counter.
 	UFUNCTION()
 	UlxRootCanvasSlot* AddChildToRootCanvas(UWidget* Content);
 
 	// Find children of type UserWidget.  Return them in a sorted
-	// order, with the highest Zorder first.
+	// order, with the bottom window first and the top window last.
 	TArray<UlxRootCanvasSlot*> GetSortedUserWidgets();
 
-	// Return the largest ZOrder across all slots, or 0 if empty.
-	// Used as the basis for placing new widgets on top.
-	int32 GetMaxZOrder() const;
+	// Return the highest-priority UserWidget slot, or nullptr if there are none.
+	UlxRootCanvasSlot* GetTopWidget();
 
 	// This function updates several window-management-related properties
 	// which are stored in the UserWidget and the lxRootCanvasSlot.  Note that
@@ -119,6 +109,17 @@ public:
 		bool ShowPointer, bool BlockInput, bool EnableEnhancedInput,
 		bool BringToFront, UWidget *DesiredFocusWidget);
 
+	// Bring the widget to the front of the Z order within its group.
+	// This is implemented using the BringToFront counter.
+	UFUNCTION(BlueprintCallable, Category = "Luprex|Window Management",
+		meta = (DefaultToSelf = "Widget"))
+	static void BringToFront(class UUserWidget *Widget);
+
+	// Recompute and apply ZOrder values for all slots based on ShowPointer and
+	// BringToFrontCount.  Must be called from a context where SetZOrder is safe -
+	// i.e. not during OnConstruct.
+	void UpdateZOrders();
+
 	// Fetch the UlxRootCanvasSlot for a widget that is parented to a
 	// UlxRootCanvasPanel. Returns nullptr via the WrongType exec pin
 	// if the widget isn't a root widget (no slot, or slot is not a
@@ -129,8 +130,17 @@ public:
 
 protected:
 
-	// UPanelWidget
+	// We inherit most of our code from CanvasPanel.   This causes the
+	// CanvasPanel code to allocate slots of type UlxRootCanvasSlot.
 	virtual UClass* GetSlotClass() const override;
+
+	// We override OnSlotAdded in order to be able to be able to
+	// fully finish initializing the slot before the widget
+	// OnConstruct method has a chance to execute.
+	virtual void OnSlotAdded(UPanelSlot* InSlot) override;
+
+	// Monotonic counter incremented each time any slot is brought to front.
+	int32 BringToFrontCounter = 0;
 };