Try to fix #2522 for a puck.

lib/bullet/src/BulletCollision/NarrowPhaseCollision/btGjkPairDetector.cpp

@@ -17,6 +17,10 @@ subject to the following restrictions:
 #include "BulletCollision/CollisionShapes/btConvexShape.h"
 #include "BulletCollision/NarrowPhaseCollision/btSimplexSolverInterface.h"
 #include "BulletCollision/NarrowPhaseCollision/btConvexPenetrationDepthSolver.h"
+#include "BulletCollision/CollisionDispatch/btManifoldResult.h"
+#include "BulletCollision/CollisionDispatch/btCollisionObject.h"
+#include "BulletCollision/CollisionShapes/btTriangleShape.h"
+#include "BulletDynamics/Dynamics/btRigidBody.h"
 
 
 
@@ -441,6 +445,32 @@ void btGjkPairDetector::getClosestPointsNonVirtual(const ClosestPointInput& inpu
 		m_cachedSeparatingAxis = normalInB;
 		m_cachedSeparatingDistance = distance;
 
+        // In some cases the normalInB is incorrect with a static triangle
+        // mesh (i.e. it's the connection point between puck and triangle, not
+        // the normal from the triangle - which is fine if the triangle mesh
+        // is a dynamic rigid body (i.e. can be pushed away), but appears to
+        // be wrong in case of a static body - it causes #2522 (puck suddenly
+        // pushed in air). So in case of a static triangle mesh, recompute
+        // the normal just based on the triangle mesh:
+        const btManifoldResult *mani = dynamic_cast<btManifoldResult*>(&output);
+        if(mani)
+        {
+            // Find the triangle:
+            const btCollisionObject *b = mani->getBody0Internal();
+            const btTriangleShape *tri = dynamic_cast<const btTriangleShape*>(b->getCollisionShape());
+            if(!tri)
+            {
+                b   = mani->getBody1Internal();
+                tri = dynamic_cast<const btTriangleShape*>(b->getCollisionShape());
+            }
+            // If we have a triangle and it is static (mass=0), recompute the normal
+            if(tri && btRigidBody::upcast(b)->getInvMass()==0)
+            {
+                normalInB = (tri->m_vertices1[1]-tri->m_vertices1[0])
+                      .cross(tri->m_vertices1[2]-tri->m_vertices1[0]);
+                normalInB.normalize();;
+            }
+        }
 		output.addContactPoint(
 			normalInB,
 			pointOnB+positionOffset,