I hear fire engines outside...

Pick away!

Actually I found the problem in something else... And, yes, I put the CreateWindow in the example as I was ferreting things out from a much bigger piece of code so that I could see the problem happening explicitly from the window appearing and "bugging" me.

But your point is well taken.

And here is a simplification (I think ;-) ) that removes the pure function and focuses on the violation of the HoldAll:

In[1]:= Attributes[g] = {HoldAll};

In[2]:= g[Sin[2.]]

Out[2]= g[Sin[2.]]

In[3]:= MapAt[f, g[Sin[2.]], 1]

Out[3]= g[f[0.909297]]