To properly generate modelStudio in R Markdown HTML, add
additional CSS into the .Rmd file.
One can either make the space wider, e.g.:
<style type="text/css">
.body {
  max-width: 1200px;
}
.main-container {
  max-width: 1200px;
}
</style>Or/and move the dashboard to the left, e.g.:
<style type="text/css">
.r2d3 {
  position: relative !important;
  left: -180px !important;
}
</style>Then, proceed to generate the modelStudio:
library("DALEX")
library("ranger")
library("modelStudio")
# fit a model
model <- ranger(score ~., data = happiness_train)
# create an explainer for the model    
explainer <- explain(model,
                     data = happiness_test,
                     y = happiness_test$score,
                     label = "Random Forest",
                     verbose = FALSE)
# make a studio for the model
modelStudio(explainer)