gtk3: Add missing titlebar border-radius rules

These were added in ancient upstream commit [1], and seem like they
could be beneficial.

[1] https://gitlab.gnome.org/GNOME/gtk/-/commit/79bf5b859289d3fe7d6be450ce5bb8eb14eed2dc

common/gtk-3.0/sass/_common.scss

@@ -1167,10 +1167,24 @@ headerbar,
  separator.titlebutton { @extend %header_separator; }
 }
 
-.titlebar {
+.titlebar,
+.background .titlebar {
  border-radius: $_header_radius $_header_radius 0 0;
 }
 
+.background.tiled .titlebar,
+.background.tiled-top .titlebar,
+.background.tiled-right .titlebar,
+.background.tiled-bottom .titlebar,
+.background.tiled-left .titlebar,
+.background.maximized .titlebar,
+.background.solid-csd .titlebar {
+ &:backdrop, & {
+ border-top-left-radius: 0;
+ border-top-right-radius: 0;
+ }
+}
+
 headerbar {
  @extend %header_widgets;
 
@@ -1217,6 +1231,25 @@ headerbar {
  }
  }
  }
+
+ window.tiled &,
+ window.tiled-top &,
+ window.tiled-right &,
+ window.tiled-bottom &,
+ window.tiled-left &,
+ window.maximized &,
+ window.fullscreen &,
+ window.solid-csd & {
+ &, &:backdrop {
+ &,
+ &:first-child,
+ &:last-child,
+ &:only-child {
+ border-top-left-radius: 0;
+ border-top-right-radius: 0;
+ }
+ }
+ }
 }
 
 // Fixes split headerbars too