diff --git a/src/App.css b/src/App.css
index 60dc3d9..087ef55 100644
--- a/src/App.css
+++ b/src/App.css
@@ -79,3 +79,11 @@
   min-height: 0;
   position: relative;
 }
+
+/* 2px padding on each leaf slot creates a 4px gap between adjacent panes —
+   so per-pane borders (idle red, active blue, broadcasting orange) read as
+   distinct rectangles instead of merging into a continuous grid pattern. */
+.leaf-slot {
+  padding: 2px;
+  box-sizing: border-box;
+}