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)