Perhaps you can try this:
htmltools::div(style = "display:inline-block", plotlyOutput("y", width = 250, height = 400))
which should serve the same purpose as plotlyOutput("y", width = 250, height = 400, inline = T)
Perhaps you can try this:
htmltools::div(style = "display:inline-block", plotlyOutput("y", width = 250, height = 400))
which should serve the same purpose as plotlyOutput("y", width = 250, height = 400, inline = T)