update

src/_sass/gtk/_common-3.20.scss

@@ -1830,12 +1830,29 @@ notebook {
     &, > stack {
       border-radius: $corner-radius / 2;
     }
+
+    > stack {
+      scrolledwindow.frame {
+        border: none;
+      }
+
+      .inline-toolbar {
+        border-width: 1px 0 0;
+        border-radius: 0 0 $corner-radius / 2 $corner-radius / 2;
+      }
+    }
   }
 
   .background.csd &.frame {
     &, > stack {
       border-radius: $corner-radius;
     }
+
+    > stack {
+      .inline-toolbar {
+        border-radius: 0 0 $corner-radius $corner-radius;
+      }
+    }
   }
 
   > stack {

src/gtk/3.0/gtk-Compact.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 5px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 5px 5px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 10px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 10px 10px;
+}
+
 notebook > stack {
   background-color: transparent;
 }

src/gtk/3.0/gtk-Dark-Compact.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 5px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 5px 5px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 10px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 10px 10px;
+}
+
 notebook > stack {
   background-color: transparent;
 }

src/gtk/3.0/gtk-Dark.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 6px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 6px 6px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 12px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 12px 12px;
+}
+
 notebook > stack {
   background-color: transparent;
 }

src/gtk/3.0/gtk-Light-Compact.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 5px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 5px 5px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 10px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 10px 10px;
+}
+
 notebook > stack {
   background-color: transparent;
 }

src/gtk/3.0/gtk-Light.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 6px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 6px 6px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 12px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 12px 12px;
+}
+
 notebook > stack {
   background-color: transparent;
 }

src/gtk/3.0/gtk.css

@@ -2103,10 +2103,23 @@ notebook.frame, notebook.frame > stack {
   border-radius: 6px;
 }
 
+notebook.frame > stack scrolledwindow.frame {
+  border: none;
+}
+
+notebook.frame > stack .inline-toolbar {
+  border-width: 1px 0 0;
+  border-radius: 0 0 6px 6px;
+}
+
 .background.csd notebook.frame, .background.csd notebook.frame > stack {
   border-radius: 12px;
 }
 
+.background.csd notebook.frame > stack .inline-toolbar {
+  border-radius: 0 0 12px 12px;
+}
+
 notebook > stack {
   background-color: transparent;
 }