fix(experimental): keep the sticky header flush with the scrollport

packages/experimental/client-ui-perf/src/client/perf.module.css

@@ -19,7 +19,13 @@
   height: 100%;
   min-height: 0;
   overflow-y: auto;
-  padding: 12px 20px calc(var(--dsh-composer-height, 152px) + 16px);
+  /* No top padding: a sticky header sits at the scrollport top, and padding
+     here would leave a strip where rows scroll above it. */
+  padding: 0 20px calc(var(--dsh-composer-height, 152px) + 16px);
+}
+
+.scroll > :first-child {
+  margin-top: 12px;
 }
 
 .empty {