Thanks, MrDeath.

The child widget relies on another library (VTK), unless you have this library, otherwise, you will not be able to build.

It seems that my code is fine with Linux. Moreover, it works with previous version of Qt, version 4.4 if remember correctly. It has the same problem with even earlier version, i.e., 4.3.

I guess either Qt or QVTKWidget caused this problem, but I'm not sure what could be the real cause of it.