Core/Movement: Fix some undermap issues with random movement/fear/blink (#22937)

* Core/Movement:
- Only move to point if there is a path that is not a shortcut (which will make the unit move through terrain)
- Added new function to check if there is a vmap floor without search distance
- Units that can fly, are underground but far above the vmap floor will stay underground (bronze drakes in tanaris)
- Don't remove PATHFIND_SHORTCUT from path type in some cases

* Core/Object: Ignore UpdateAllowedPositionZ for flying units.

- This will make flying units go through mountains instead of going to the top and back to the bottom to reach you.

* Core/Object: Revert some changes and let MovePositionToFirstCollision deal with a position without ground

* Missing groundZ change for objects on transport

* use CanFly instead of IsFlying

(cherry picked from commit 9fcbd8f15d)

src/server/game/Entities/Object/Object.cpp

@@ -1250,11 +1250,16 @@ void WorldObject::UpdateGroundPositionZ(float x, float y, float &z) const
         z = new_z + (isType(TYPEMASK_UNIT) ? static_cast<Unit const*>(this)->GetHoverOffset() : 0.0f);
 }
 
-void WorldObject::UpdateAllowedPositionZ(float x, float y, float &z) const
+void WorldObject::UpdateAllowedPositionZ(float x, float y, float &z, float* groundZ) const
 {
     // TODO: Allow transports to be part of dynamic vmap tree
     if (GetTransport())
+    {
+        if (groundZ)
+            *groundZ = z;
+
         return;
+    }
 
     if (Unit const* unit = ToUnit())
     {
@@ -1280,12 +1285,18 @@ void WorldObject::UpdateAllowedPositionZ(float x, float y, float &z) const
                 else if (z < ground_z)
                     z = ground_z;
             }
+
+            if (groundZ)
+                *groundZ = ground_z;
         }
         else
         {
             float ground_z = GetMapHeight(x, y, z) + unit->GetHoverOffset();
             if (z < ground_z)
                 z = ground_z;
+
+            if (groundZ)
+               *groundZ = ground_z;
         }
     }
     else
@@ -1293,6 +1304,9 @@ void WorldObject::UpdateAllowedPositionZ(float x, float y, float &z) const
         float ground_z = GetMapHeight(x, y, z);
         if (ground_z > INVALID_HEIGHT)
             z = ground_z;
+
+        if (groundZ)
+            *groundZ = ground_z;
     }
 }
 
@@ -3102,10 +3116,27 @@ void WorldObject::MovePositionToFirstCollision(Position &pos, float dist, float
         }
     }
 
+    float groundZ = VMAP_INVALID_HEIGHT_VALUE;
     Trinity::NormalizeMapCoord(pos.m_positionX);
     Trinity::NormalizeMapCoord(pos.m_positionY);
-    UpdateAllowedPositionZ(destx, desty, pos.m_positionZ);
+    UpdateAllowedPositionZ(destx, desty, pos.m_positionZ, &groundZ);
     pos.SetOrientation(GetOrientation());
+
+    // position has no ground under it (or is too far away)
+    if (groundZ <= INVALID_HEIGHT)
+    {
+        if (Unit const* unit = ToUnit())
+        {
+            // unit can fly, ignore.
+            if (unit->CanFly())
+                return;
+
+            // fall back to gridHeight if any
+            float gridHeight = GetMap()->GetGridHeight(GetPhaseShift(), pos.m_positionX, pos.m_positionY);
+            if (gridHeight > INVALID_HEIGHT)
+                pos.m_positionZ = gridHeight + unit->GetHoverOffset();
+        }
+    }
 }
 
 void WorldObject::PlayDistanceSound(uint32 soundId, Player* target /*= nullptr*/)