Prevent rounded corners in the middle of a BorderBox

[HDinger]

What are you trying to accomplish?

In a Primer::Beta::BorderBox there are constellations in which there are rounded corners somewhere in the middle. For example

render(Primer::Beta::BorderBox.new) do |component|
  component.with_header { "Header" }
  component.with_body { "Body" }
  component.with_row { "Row one" }
  component.with_row { "Row two" }
end

Screenshots

List the issues that this change affects.

Closes #2684

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Merge checklist

• Added/updated tests
• Added/updated documentation
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Latest commit: c3c60f0

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@primer/view-components	Patch

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[jonrohan]

An issue with this selector is we often have hidden elements at the end of the rows in our markup

[HDinger]

I tried to re-create such an example, but failed. When looking at the html of the BorderBox, there is no hidden element defined, and the slot definitions do not accept blocks as arguments either.

<%= render Primer::BaseComponent.new(**@system_arguments) do %>
  <%= header %>
  <%= body %>
  <% if rows.any? %>
    <%= render Primer::BaseComponent.new(**@list_arguments) do %>
      <% rows.each do |row| %>
        <%= row %>
      <% end %>
    <% end %>
  <% end %>
  <%= footer %>
<% end %>

Could you give me a hint, on how to create an example with hidden elements at the end of the rows?