The IME window is sometimes flickering. How can I improve this behavior?

You should call GXEnableIme() in your InitInstance method.

Example:

a) Insert      
      GX_IMPLEMENT_IME()
   in your application class .CPP file.
b) Insert 
      GXEnableIme();
   into your InitInstance method.
c) Change the Project Settings. Open Link and enter 
      ime32.lib
   in the object library/modules edit box.