apprt/gtk-ng: split separator styling

src/apprt/gtk-ng/css/style-dark.css

@@ -1,3 +1,8 @@
 .transparent {
   background-color: transparent;
 }
+
+.window .split paned > separator {
+  background-color: rgba(36, 36, 36, 1);
+  background-clip: content-box;
+}

src/apprt/gtk-ng/css/style.css

@@ -114,3 +114,26 @@ label.resize-overlay {
   margin-left: 4px;
   margin-right: 8px;
 }
+
+/*
+ * Splits
+ */
+
+.window .split paned > separator {
+  background-color: rgba(250, 250, 250, 1);
+  background-clip: content-box;
+
+  /* This works around the oversized drag area for the right side of GtkPaned.
+   *
+   * Upstream Gtk issue:
+   * https://gitlab.gnome.org/GNOME/gtk/-/issues/4484#note_2362002
+   *
+   * Ghostty issue:
+   * https://github.com/ghostty-org/ghostty/issues/3020
+   *
+   * Without this, it's not possible to select the first character on the
+   * right-hand side of a split.
+   */
+  margin: 0;
+  padding: 0;
+}

src/apprt/gtk-ng/ui/1.5/split-tree-split.blp

@@ -2,6 +2,10 @@ using Gtk 4.0;
 using Adw 1;
 
 template $GhosttySplitTreeSplit: Adw.Bin {
+  styles [
+    "split",
+  ]
+
   // The double-nesting is required due to a GTK bug where you can't
   // bind the first child of a builder layout. If you do, you get a double
   // dispose. Easiest way to see that is simply remove this and see the