Comment 31 for bug 1986450

DING developer's response is basically that they clear the "decorated" property of the window, and the theme should handle that.

Based on this, I think the correct solution for the Ambiance/Radiance themes is to further restrict the offending block with the ".decorated" selector, i.e.

  .background.decorated:backdrop {
      color: @backdrop_fg_color;
      box-shadow: inset -1px 0 shade (@bg_color, 0.94);
  }

or for the user's own gtk.css to select for its negated ":not(.decorated)" for the workaround:

  .background:not(.decorated):backdrop {
      box-shadow: none;
  }