- do not process subsectors if rendering a stacked sector portal and the clipper is set to 'blocked', i.e. no visible parts are present.

This solves the most severe occurences of sprites being drawn in front of a portal's contents. It is not a full fix, though, there's still some extreme cases where portals may glitch if some stuff gets between the camera and the actual portal area.
Normally this has to perform a full check of the subsector against the portal's camera-facing linedefs but that's too costly for those rare cases where it may be an issue.

src/gl/scene/gl_bsp.cpp

@@ -79,6 +79,7 @@ static void UnclipSubsector(subsector_t *sub)
 		if (startAngle-endAngle >= ANGLE_180)  
 		{
 			clipper.SafeRemoveClipRange(startAngle, endAngle);
+			clipper.SetBlocked(false);
 		}
 		seg++;
 	}
@@ -444,6 +445,7 @@ static void DoSubsector(subsector_t * sub)
 		// range this subsector spans before going on.
 		UnclipSubsector(sub);
 	}
+	if (clipper.IsBlocked()) return;	// if we are inside a stacked sector portal which hasn't unclipped anything yet.
 
 	fakesector=gl_FakeFlat(sector, &fake, false);
 

src/gl/scene/gl_clipper.cpp

@@ -92,6 +92,7 @@ void Clipper::Clear()
 	ClipNode *node = cliphead;
 	ClipNode *temp;
 	
+	blocked = false;
 	while (node != NULL)
 	{
 		temp = node;

src/gl/scene/gl_clipper.h

@@ -54,6 +54,7 @@ class Clipper
 	ClipNode * clipnodes;
 	ClipNode * cliphead;
 	ClipNode * silhouette;	// will be preserved even when RemoveClipRange is called
+	bool blocked;
 
 	static angle_t AngleToPseudo(angle_t ang);
 	bool IsRangeVisible(angle_t startangle, angle_t endangle);
@@ -68,6 +69,7 @@ public:
 
 	Clipper()
 	{
+		blocked = false;
 		clipnodes=cliphead=NULL;
 	}
 
@@ -129,6 +131,16 @@ public:
 		SafeRemoveClipRange(AngleToPseudo(startangle), AngleToPseudo(endangle));
 	}
 
+	void SetBlocked(bool on)
+	{
+		blocked = on;
+	}
+
+	bool IsBlocked() const
+	{
+		return blocked;
+	}
+
 	bool CheckBox(const float *bspcoord);
 };
 

src/gl/scene/gl_portal.cpp

@@ -763,6 +763,7 @@ void GLSectorStackPortal::DrawContents()
 	if (!(gl_drawinfo->ss_renderflags[sub - ::subsectors] & SSRF_SEEN))
 	{
 		clipper.SafeAddClipRange(0, ANGLE_MAX);
+		clipper.SetBlocked(true);
 	}
 
 	GLRenderer->DrawScene(DM_PORTAL);