hbox() with two PlotlyJS plots emits StackOverflowError

[bjarthur]

create an interactive graphic with two plots throws an error. one button and one plot works, but not two plots.

julia> using Interact, Mux, PlotlyJS, WebIO

julia> b = button()

julia> p1 = plot(rand(10));

julia> p2 = plot(rand(10));

julia> ui = hbox(p1,b);     ### this works

julia> ui = hbox(p1,p2)    ### this does not
ERROR: StackOverflowError:

julia> WebIO.webio_serve(page("/", req -> ui), 5006)

xref https://github.com/sglyon/PlotlyJS.jl/issues/311