update

src/_sass/gtk/_common-3.20.scss

@@ -3581,7 +3581,10 @@ decoration {
   .popup & { box-shadow: none; }
 
   // server-side decorations as used by mutter
-  .ssd & { box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16); } // just doing borders, wm draws actual shadows
+  .ssd & {
+    border-radius: $window-radius $window-radius 0 0;
+    box-shadow: none; // just doing borders, wm draws actual shadows
+  }
 
   .csd.popup & {
     border-radius: $corner-radius;

src/gtk/3.0/gtk-compact.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {

src/gtk/3.0/gtk-dark-compact.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {

src/gtk/3.0/gtk-dark.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {

src/gtk/3.0/gtk-light-compact.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {

src/gtk/3.0/gtk-light.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {

src/gtk/3.0/gtk.css

@@ -5000,7 +5000,8 @@ decoration:backdrop {
 }
 
 .ssd decoration {
-  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.16);
+  border-radius: 18px 18px 0 0;
+  box-shadow: none;
 }
 
 .csd.popup decoration {