Started using more native rounded corners

src/browser/base/content/zen-styles/zen-browser-container.css

@@ -6,7 +6,18 @@
 :root:not([inDOMFullscreen='true']):not([chromehidden~='location']):not([chromehidden~='toolbar']) {
   & #tabbrowser-tabbox #tabbrowser-tabpanels .browserSidebarContainer {
     width: -moz-available;
-    border-radius: var(--zen-webview-border-radius, var(--zen-border-radius));
+    --zen-native-content-radius: env(-moz-gtk-csd-titlebar-radius, var(--zen-border-radius));
+    border-radius: var(
+      --zen-webview-border-radius,
+      /* Inner radius calculation:
+       * 1. If the native radius - the separation is less than 4px, use 4px.
+       * 2. Otherwise, use the the calculated value (inner radius = outer radius - separation).
+      */
+      max(
+        4px,
+        calc(var(--zen-native-content-radius) - var(--zen-element-separation)),
+      )
+    );
     position: relative;
 
     /* For glance */