Replicate RMVX/VXA's broken sprite Z sorting implementation

Fixes Window_NameBox visual appearance in Skyborn.

Also nuke the second SceneElement constructor that has been
obsolete since the Tilemap mapViewport rewrite.

src/scene.cpp

@@ -105,26 +105,13 @@ void Scene::composite()
 }
 
 
-SceneElement::SceneElement(Scene &scene, int z, bool isSprite)
+SceneElement::SceneElement(Scene &scene, int z, int spriteY)
     : link(this),
       creationStamp(shState->genTimeStamp()),
       z(z),
       visible(true),
       scene(&scene),
-      spriteY(0),
-      isSprite(isSprite)
-{
-	scene.insert(*this);
-}
-
-SceneElement::SceneElement(Scene &scene, int z, unsigned int cStamp)
-    : link(this),
-      creationStamp(cStamp),
-      z(z),
-      visible(true),
-      scene(&scene),
-      spriteY(0),
-      isSprite(false)
+      spriteY(spriteY)
 {
 	scene.insert(*this);
 }
@@ -184,11 +171,11 @@ bool SceneElement::operator<(const SceneElement &o) const
 				/* RGSS2: If two sprites' Z values collide,
 				 * their Y coordinates decide draw order. Only
 				 * on equal Y does the creation time take effect */
-				if (isSprite && o.isSprite && spriteY != o.spriteY)
+				if (spriteY != o.spriteY)
 					return (spriteY < o.spriteY);
 			}
 
-			return (creationStamp <= o.creationStamp);
+			return (creationStamp < o.creationStamp);
 		}
 
 		return true;

src/scene.h

@@ -71,8 +71,7 @@ protected:
 class SceneElement
 {
 public:
-	SceneElement(Scene &scene, int z = 0, bool isSprite = false);
-	SceneElement(Scene &scene, int z, unsigned int cStamp);
+	SceneElement(Scene &scene, int z = 0, int spriteY = 0);
 	virtual ~SceneElement();
 
 	void setScene(Scene &scene);
@@ -122,8 +121,17 @@ protected:
 	friend struct TilemapPrivate;
 
 private:
+
+	/* RGSS2 introduced an enhanced type of Z ordering: sprites with
+	 * the same Z are first ordered by their Y value (higher Y = closer
+	 * to player) and then by creation time. However, the Enterbrain devs
+	 * botched their implementation, and now every other scene element
+	 * subclass is sorted as if it was a sprite with a fixed Y of 0.
+	 * In RGSS3, they tried to fix this for the Window class, badly. It
+	 * now behaves as if it was a sprite with fixed Y of infinity. This
+	 * means that sprites created _after_ a window with the same Z will
+	 * still always be displayed below said window. */
 	int spriteY;
-	const bool isSprite;
 };
 
 #endif // SCENE_H

src/sprite.cpp

@@ -290,7 +290,7 @@ struct SpritePrivate
 };
 
 Sprite::Sprite(Viewport *viewport)
-	: ViewportElement(viewport, 0, true)
+    : ViewportElement(viewport)
 {
 	p = new SpritePrivate;
 	onGeometryChange(scene->getGeometry());

src/tilemap.cpp

@@ -313,14 +313,6 @@ struct TilemapPrivate
 		size_t activeLayers;
 		Scene::Geometry sceneGeo;
 		Vec2i sceneOffset;
-
-		/* The ground and zlayer elements' creationStamp
-		 * should be aquired once (at Tilemap construction)
-		 * instead of regenerated everytime the elements are
-		 * (re)created. ZLayers can share one stamp because
-		 * their z always differs anway */
-		unsigned int groundStamp;
-		unsigned int zlayerStamp;
 	} elem;
 
 	/* Affected by: autotiles, tileset */
@@ -398,9 +390,6 @@ struct TilemapPrivate
 		flash.quadCount = 0;
 		flash.alphaIdx = 0;
 
-		elem.groundStamp = shState->genTimeStamp();
-		elem.zlayerStamp = shState->genTimeStamp();
-
 		elem.ground = new GroundLayer(this, viewport);
 
 		for (size_t i = 0; i < zlayersMax; ++i)
@@ -1077,7 +1066,7 @@ struct TilemapPrivate
 };
 
 GroundLayer::GroundLayer(TilemapPrivate *p, Viewport *viewport)
-    : ViewportElement(viewport, 0, p->elem.groundStamp),
+    : ViewportElement(viewport, 0),
       vboCount(0),
       p(p)
 {
@@ -1139,7 +1128,7 @@ void GroundLayer::onGeometryChange(const Scene::Geometry &geo)
 }
 
 ZLayer::ZLayer(TilemapPrivate *p, Viewport *viewport)
-    : ViewportElement(viewport, 0, p->elem.zlayerStamp),
+    : ViewportElement(viewport, 0),
       index(0),
       vboOffset(0),
       vboCount(0),

src/viewport.cpp

@@ -214,15 +214,11 @@ void Viewport::onGeometryChange(const Geometry &geo)
 }
 
 
-ViewportElement::ViewportElement(Viewport *viewport, int z, bool isSprite)
-    : SceneElement(viewport ? *viewport : *shState->screen(), z, isSprite),
+ViewportElement::ViewportElement(Viewport *viewport, int z, int spriteY)
+    : SceneElement(viewport ? *viewport : *shState->screen(), z, spriteY),
       m_viewport(viewport)
 {}
 
-ViewportElement::ViewportElement(Viewport *viewport, int z, unsigned int cStamp)
-    : SceneElement(viewport ? *viewport : *shState->screen(), z, cStamp)
-{}
-
 Viewport *ViewportElement::getViewport() const
 {
 	return m_viewport;

src/viewport.h

@@ -61,8 +61,7 @@ private:
 class ViewportElement : public SceneElement
 {
 public:
-	ViewportElement(Viewport *viewport = 0, int z = 0, bool isSprite = false);
-	ViewportElement(Viewport *viewport, int z, unsigned int cStamp);
+	ViewportElement(Viewport *viewport = 0, int z = 0, int spriteY = 0);
 
 	DECL_ATTR( Viewport,  Viewport* )
 

src/windowvx.cpp

@@ -32,12 +32,14 @@
 #include "glstate.h"
 #include "shader.h"
 
+#include <limits>
 #include <algorithm>
 #include <sigc++/connection.h>
 
 #define DEF_Z         (rgssVer >= 3 ? 100 :   0)
 #define DEF_PADDING   (rgssVer >= 3 ?  12 :  16)
 #define DEF_BACK_OPAC (rgssVer >= 3 ? 192 : 255)
+#define DEF_SPRITE_Y  (rgssVer >= 3 ? std::numeric_limits<int>::max() : 0) /* See scene.h */
 
 template<typename T>
 struct Sides
@@ -811,14 +813,14 @@ struct WindowVXPrivate
 };
 
 WindowVX::WindowVX(Viewport *viewport)
-	: ViewportElement(viewport, DEF_Z)
+    : ViewportElement(viewport, DEF_Z, DEF_SPRITE_Y)
 {
 	p = new WindowVXPrivate(0, 0, 0, 0);
 	onGeometryChange(scene->getGeometry());
 }
 
 WindowVX::WindowVX(int x, int y, int width, int height)
-    : ViewportElement(0, DEF_Z)
+    : ViewportElement(0, DEF_Z, DEF_SPRITE_Y)
 {
 	p = new WindowVXPrivate(x, y, width, height);
 	onGeometryChange(scene->getGeometry());