testtext might be passed to DefWindowProcW in test_aw_conversion_dlgprocA, so it must have a WCHAR null at the end.
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@nsivov it's also potentially possible for a WCHAR string to go throught W->A conversion then be passed to DefWindowProcW. in that case i am not sure how to guarantee it is wide-null terminated. this doesn't current happen in our test cases though.