Propperly handle instand speed increases in the physics when rewinding.

src/karts/kart.cpp

@@ -1168,6 +1168,9 @@ void Kart::eliminate()
  */
 void Kart::update(float dt)
 {
+    // Reset any instand speed increase in the bullet kart
+    m_vehicle->resetInstantSpeed();
+
     // update star effect (call will do nothing if stars are not activated)
     m_stars_effect->update(dt);
 

src/karts/kart_rewinder.cpp

@@ -74,7 +74,7 @@ BareNetworkString* KartRewinder::saveState() const
     buffer->add(body->getLinearVelocity());
     buffer->add(body->getAngularVelocity());
     buffer->addUInt8(m_has_started);   // necessary for startup speed boost
-    buffer->addFloat(m_vehicle->getZipperSpeed());
+    buffer->addFloat(m_vehicle->getInstantSpeedIncrease());
 
     // 2) Steering and other player controls
     // -------------------------------------
@@ -116,6 +116,9 @@ void KartRewinder::rewindToState(BareNetworkString *buffer)
     // This function also reads the velocity, so it must be called
     // after the velocities are set
     body->proceedToTransform(t);
+    // Update kart transform in case that there are access to its value
+    // before Moveable::update() is called (which updates the transform)
+    setTrans(t);
     m_has_started = buffer->getUInt8()!=0;   // necessary for startup speed boost
     m_vehicle->instantSpeedIncreaseTo(buffer->getFloat());
 

src/physics/btKart.cpp

@@ -1062,6 +1062,11 @@ void btKart::setSliding(bool active)
  */
 void btKart::instantSpeedIncreaseTo(float speed)
 {
+    // Avoid that a speed 'increase' might cause a slowdown
+    if (m_chassisBody->getLinearVelocity().length2() > speed*speed)
+    {
+        return;
+    }
     m_zipper_active = speed > 0;
     m_zipper_speed  = speed;
 }   // activateZipper

src/physics/btKart.hpp

@@ -276,7 +276,9 @@ public:
     }   // setTimedTorque
     // ------------------------------------------------------------------------
     /** Returns the current zipper speed. */
-    float getZipperSpeed() const { return m_zipper_speed; }
+    float getInstantSpeedIncrease() const { return m_zipper_speed; }
+    // ------------------------------------------------------------------------
+    void resetInstantSpeed() { m_zipper_speed = 0;  }
 };   // class btKart
 
 #endif //BT_KART_HPP