Capybara not seeing updates to the DOM made by Javascript - ruby-on-rails

Question

I'm running Rails 5.x, with, Cucumber, Siteprism and Capybara through chromedriver. Most things work except..
I have a tiny bit of javascript that changes the class on an element in response to an event. But Capybara never sees the change. It only ever sees the class the element has when the page initially loaded.
Using Chrome, and debugging my Cucumber steps, I can see the element has the new class, but Capybara doesn't see it.
This must be an issue other people have encountered and solved, though I can't find the right subject title.
example coffeescript
$(document).on('focus', 'tbody#item-entry > tr > td > input', (e) ->
$(#).closest('tr').addClass('focused-row')
$(#).closest('td').addClass('focused-cell')
)
example html after the focus event has been triggered
<tr class="focused-row">
<td>ignore this </td>
</tr>
The purpose is to change the background colour of the row containing an input element that has focus. It works.
But Capybara, can't see the class, but it can see any classes added when the page is loaded. e.g.
expect(siteprism_stuff.root_element['class']).to match(/focused-row/)
Ignore the SitePrism stuff, that just gets the right element. root_element is the Capybara class for the dom node.
Now I know it's getting the right Capybara element because if I change my view to put stuff in the class for each row, then it sees that perfectly OK. What it can't see is the any new class added via Coffeescript. Although it's visible in the Chrome inspector, and changes the background color of the focused row as required.

Answer 1

You're specifying an "ends with" CSS attribute selector ($=)
input[class$='form-control']
which since the class attribute for the element you're interested in
<input type="search" class="form-control form-control-sm" placeholder="" aria-controls="universitiesTable">
doesn't end with 'form-control' is correctly not matching. You probably just want to use a normal CSS class selector input.form-control if continuing to do it the way you are. Any of the following options should find the search field and fill in the data you are trying to fill in.
fill_in 'Search:', with: string
fill_in type: 'search', with: string
find(:field, type: 'search').set(string)
find('input.form-control').set(string)

Answer 2

Note: Your question is still unclear as to whether you are seeing the class added in the inspector in test mode, and whether the line color is changing while the tests are running (or whether you're only seeing that in dev mode) - This answer assumes the JS is actually running in test mode and you're seeing the line color change while the tests are running.
You don't show how you're actually triggering the focus event but I'll assume you're clicking the element. The thing to understand when working with Capybara is that the browser works asynchronously, so when something like click has been done, the actions triggered by that click have not necessarily been done yet. Because of that, whenever doing any type of expectation with page elements you should always be using the matchers provided by Capybara rather than the basic matchers provided by RSpec. The Capybara provided matchers include waiting/retrying behavior to handle the asynchronous nature of dealing with the browser. In this case, assuming siteprism_stuff.root_element is the row element then you could be doing something like
expect(siteprism_stuff.root_element).to match_css('.focused-row')
or depending on exactly how your siteprism page objects are setup you could pass the class option to the siteprism existence checker
# `page_section` and `have_row` would need to be replaced with whatever is correct for your site prism page object
expect(page_section).to have_row(class: ['.focused-row'])