that said, I was able to repair it, sufficiently to then repair it.... 
library(shiny)
library(plotly)
ui <- fluidPage(
plotlyOutput("new_plot")
)
server <- function(input, output, session) {
s <-reactive({
rv <- reactiveValues(x = NULL, y = NULL)
rv$x <- as.vector(-50:50)
rv$y <- as.vector(rv$x^2)
d <- event_data("plotly_relayout", source = "trajectory")
selected_point <- if (!is.null(d[["shapes[0].x0"]])) {
xint <- d[["shapes[0].x0"]]
xpt <- rv$x[which.min(abs(rv$x - xint))]
list(x = xpt, y = rv$y[which.min(abs(rv$x - xint))])
} else {
list(x = min(rv$x), y = rv$y[which(rv$x == min(rv$x))])
}
print(selected_point)
plot_ly(color = I("red"), source = "trajectory") %>%
add_lines(x = rv$x, y = rv$y) %>%
add_markers(x = selected_point$x, y = selected_point$y) %>%
layout(
shapes = list(
type = "line",
line = list(color = "gray", dash = "dot"),
x0 = selected_point$x,
x1 = selected_point$x,
y0 = 0,
y1 = 1,
yref = "paper"
)
) %>%
config(editable = TRUE)
})
output$new_plot <- renderPlotly({
s()
})
}
shinyApp(ui = ui, server = server)