Thanks for the good detective work.  The flash is always there, somewhere, though sometimes minimal.  SearchBase.open begins with

        if not self.top:
            self.create_widgets()
        else:
            self.top.deiconify()
            self.top.tkraise()

The first and second open()s call the first and second branch.  I am puzzled that the first does not flash, as top is created 'normal' rather than 'withdrawn'.  It also turn out that commenting out either of the two lines stops the flash.

The flash could be prevented by changing 'else' to 'else if not _utest' and adding '_utest' as a parameter and passing True, as I have elsewhere.  However, the purpose of the second open() is to take that branch for near 100% coverage, and I have previously only used _utest to avoid blocking .wait_window calls.

I think I am going to leave this alone for now, but please feel free to comment on other IDLE issues.