If you enable the adaptive transparency in panel then you maximize any app and click on "Show Desktop" widget then the panel doesn't become translucent and remains opaque.
*** This bug has been marked as a duplicate of bug 439209 ***
Not a duplicate, because bug 439209 specifies only the case of shortcut and not when you click on "Show Desktop" widget. And "Show Desktop" widget is totally different from "Minimize all Windows" widget.
still a duplicate *** This bug has been marked as a duplicate of bug 439209 ***