One way to make Rails consider the layout in an etag.

etag.rb

# By default, changes to the layout temnplate won't cause a Rails etag
# to change. I suppose this is to facilitate Turbolinks. Sometimes you
# might want to change an etag in response to changes in the layout -
# this is particularly true when working in development mode. This is
# one way to do it, there are probably other (read: likely better) ways
# to do this too.
class FooController < ApplicationController
  # There's probably some API method to get the name of the layout
  # rather than hard-coding it like I did here, but I'm too lazy to
  # look it up right now.
  etag {
    ActionView::Digestor.digest(name: "layouts/application.html.erb", finder: view_context.lookup_context)
  }
end