status-inside-urlbar: add background-color to .browserStack

This opaque background allows us to draw statuspanel behind it.
Without this patch, the statupanel could be shown on tabs
that don't yet have associated <browser> such as when restoring
a tab after startup.

chrome/status_inside_urlbar.css

@@ -84,6 +84,11 @@ As a workaround, the -moz-element background image is removed somewhat often whi
   margin-top: 0px !important;
 }
 
+/* This creates opaque layer to be shown in front of "hidden" real statuspanel */
+.browserStack{
+  background-color: var(--tabpanel-background-color);
+}
+
 /*  OPTIONAL FEATURES  */
 /* Uncomment to enable */