one more of these display: inline issues with invisiblefieldset :-( Can someone replace this problematic solution with another one - please!

after correcting the property to display:block the search button moves below the search field when the text gets longer or the block width very narrow.

.block_search_forums .invisiblefieldset {
  display: block;
}

theme/standard/styles_layout.css

@@ -1,3 +1,4 @@
+/* @override http://moodle18.userver.lan/theme/standard/styles_layout.css */
 /*******************************************************************
  styles_layout.css
 
@@ -1290,6 +1291,10 @@ a.skip-block, .skip-block {
   text-align: center;
 }
 
+.block_search_forums .invisiblefieldset {
+  display: block;
+}
+
 /***
  *** Blogs
  ***/