Suggestion: app$layout should be more flexible by allowing a comma-separated list of arguments

[daattali]

Dash layout is meant to allow a collection, so that if I want to show two components, I have to place them inside a list (although doesn't currently work #257):

app <- Dash$new()
app$layout(list(htmlDiv("foo"), htmlDiv("bar")))

A much more convenient feature would be to allow the user to pass any number of arguments, comma-separated, like so:

app <- Dash$new()
app$layout(htmlDiv("foo"), htmlDiv("bar"))

Currently the only way to make that work is by wrapping all the content inside a parent htmlDiv()

[alexcjohnson]

Also related to plotly/dash#1620