Difficulty getting htmlwidgets::onStaticRenderComplete to work with leaflet - r

Question

I am using Shiny and trying to create synced leaflet maps that work with leafletProxy. The script which should run after the leaflet widgets have rendered doesn't seem to get called.
library(shiny)
library(leaflet)
# Modified from mapview::latticeView
sync <- function (..., ncol = 2, sync = "none", sync.cursor = FALSE,
no.initial.sync = TRUE)
{
ls <- list(...)
if (length(ls) == 1)
ls <- ls[[1]]
for (i in seq(ls)) {
if (!is.null(ls[[i]]$id)) {
ls[[i]]$elementId <- ls[[i]]$id
}
#ls[[i]] <- mapview:::mapview2leaflet(ls[[i]])
if (length(ls[[i]]$dependencies) == 0) {
ls[[i]]$dependencies = list()
}
if (is.null(ls[[i]]$elementId)) {
ls[[i]]$elementId <- paste("htmlwidget", as.integer(stats::runif(1,
1, 10000)), sep = "-")
}
}
wdth <- paste0("width:", round(1/ncol * 100, 0) - 1, "%;")
styl <- paste0("display:inline;", wdth, "float:left;border-style:solid;border-color:#BEBEBE;border-width:1px 1px 1px 1px;")
tg <- lapply(seq(ls), function(i) {
htmltools::tags$div(style = styl, leafletOutput(ls[[i]]$id))
})
sync_strng <- ""
if (!is.list(sync) && sync == "all") {
sync = list(seq(ls))
}
if (is.list(sync)) {
for (i in seq(sync)) {
synci <- sync[[i]]
sync_grid <- expand.grid(synci, synci, KEEP.OUT.ATTRS = FALSE)
sync_strng <- c(sync_strng, apply(sync_grid, MARGIN = 1,
function(combo) {
if (combo[1] != combo[2]) {
return(sprintf("leaf_widgets['%s'].sync(leaf_widgets['%s'],{syncCursor: %s, noInitialSync: %s});",
ls[[combo[1]]]$elementId, ls[[combo[2]]]$elementId,
tolower(as.logical(sync.cursor)), tolower(as.logical(no.initial.sync))))
}
return("")
}))
}
}
sync_strng <- paste0(sync_strng, collapse = "\n")
tl <- htmltools::attachDependencies(htmltools::tagList(tg,
htmlwidgets::onStaticRenderComplete(paste0("
var leaf_widgets = {}; \n
Array.prototype.map.call( \n
document.querySelectorAll(\".leaflet\"),\n
function(ldiv){\n
if (HTMLWidgets.find(\"#\" + ldiv.id) && HTMLWidgets.find(\"#\" + ldiv.id).getMap()) {\n
leaf_widgets[ldiv.id] = HTMLWidgets.find(\"#\" + ldiv.id).getMap();\n
}\n }\n );\n ",
sync_strng))), mapview:::dependencyLeafletsync())
return(htmltools::browsable(tl))
}
ui <- function(id) {
uiOutput("maps")
}
server <- function(input,output,session) {
map1 <- leaflet() %>% addTiles()
map2 <- leaflet() %>% addTiles()
output$map1 <- renderLeaflet(map1)
output$map2 <- renderLeaflet(map2)
output$maps <- renderUI({
sync2(leafletProxy('map1'), leafletProxy('map2'), sync = "all", sync.cursor = TRUE, no.initial.sync = FALSE)
})
}
runApp(shinyApp(ui, server), launch.browser=TRUE)
When the page loads, the maps are not synced, but if I go into the console and execute window.HTMLWidgets.staticRender() the post render code will run and the maps will be synced. What is causing this?