The closest question to my question was this, on which I have answered also.
I have a similar question but may have a far more user impact.
When I type input$abc the code completion in server.R will usually prompt me with all ui.R input variables starting with abc e.g. selectInput(id = 'abc123'...) or checkboxInput(id = 'abcxyx',...). But if the input is an action button (e.g. actionButton(id = 'abc456',...) ) it does not appear in the autocorrect prompt.
Is this a bug where rstudio IDE dev forgot to add actionButton and its variations (e.g. actionBttn) in the list of auto complete variables?
Can I request the IDE developers to look into this?