Steps to reproduce:
This is reproducible in vanilla Jboss author instance also.
FYI, Every time business has to clear cache to see sidekick while switching UI. this issue follows from AEM 6.1
Solved! Go to Solution.
This post on stackoverflow has potentially a better answer : cq5 - How to get sidekick in AEM 6.3? - Stack Overflow
Vijay suggests : "Sidekick is available in AEM 6.3 with Classic UI mode as exactly same as AEM 6.1 and AEM 6.2. Check the init.jsp is added to your page component. That might be one of the reason."