Investigate discoverability of header bar icon buttons
Previously we had issues with Software users not noticing the search button - GNOME/gnome-software#986 . More recently, @Udo_I identified a similar issue in Clocks (see issue 3 in #60 (closed)). It would be interesting to perform some further tests on this design pattern, to see how common the issue is, and whether there are any other contributing factors (such as the width of the window).