gtk4: Do not draw drop shadows on tiled windows

Only draw the 1px border, and transparent shadow for resizing.
This is what default GTK theme does as well [1][2][3].

[1] https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3179
[2] https://gitlab.gnome.org/GNOME/gtk/-/commit/7cc31d81f58b89106c50ff5f1a4c2a026a9413e3
[3] https://gitlab.gnome.org/GNOME/gtk/-/commit/56f6ac5fcc42e18989ef8122a93bfc4d1ef09fd0

common/gtk-4.0/sass/_common.scss

@@ -4014,6 +4014,13 @@ window {
   &.tiled-right,
   &.tiled-bottom {
     border-radius: 0;
+    box-shadow: 0 0 0 1px if($variant == 'dark' or $variant == 'darker', darken($header_bg, 7%), $_wm_border),
+                0 0 0 20px transparent; //transparent control workaround -- GTK issue #3670
+
+    &:backdrop {
+      box-shadow: 0 0 0 1px if($variant == 'dark' or $variant == 'darker', darken($header_bg, 7%), $_wm_border),
+                  0 0 0 20px transparent; // GTK issue #3670
+    }
   }
 
   &.popup { border-radius: 0; box-shadow: none; }