gtk3 (3.13.5) under windows with semi transparent window border

[Andrea Zagli <azagli libero it>]

i'm trying gtk+3 3.13.5 under windows, with msys2

adwaita theme looks very nice under windows; and at this point gtk+  
seems very stable also under windows

but i got windows with semi transparent border and title

what could it be?

thanks in advance

and thanks for your efforts with gtk+ under windows

PS: i just compiled evince 3.13.3 and it works perfectly