Stick the commandbar to the top of the seconday panel. (firefox-devtools#2128)

Author: Garbee

src/components/SecondaryPanes/CommandBar.css

@@ -4,6 +4,10 @@
   display: flex;
   height: 30px;
   overflow: hidden;
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  background-color: var(--theme-body-background);
 }
 
 .command-bar > button {