Set the issuable sidebar to remain collapsed on mobile devices

app/assets/javascripts/main.js

@@ -211,6 +211,14 @@ $(function () {
     }
   });
 
+  if (bootstrapBreakpoint === 'xs') {
+    const $rightSidebar = $('aside.right-sidebar, .page-with-sidebar');
+
+    $rightSidebar
+      .removeClass('right-sidebar-expanded')
+      .addClass('right-sidebar-collapsed');
+  }
+
   // prevent default action for disabled buttons
   $('.btn').click(function(e) {
     if ($(this).hasClass('disabled')) {

changelogs/unreleased/right-sidebar-closed-default-mobile.yml

+---
+title: Set the issuable sidebar to remain closed for mobile devices
+merge_request:
+author:

spec/features/issues/issue_sidebar_spec.rb

@@ -120,6 +120,20 @@ feature 'Issue Sidebar', feature: true do
     end
   end
 
+  context 'as a allowed mobile user', js: true do
+    before do
+      project.team << [user, :developer]
+      resize_screen_xs
+      visit_issue(project, issue)
+    end
+
+    context 'mobile sidebar' do
+      it 'collapses the sidebar for small screens' do
+        expect(page).not_to have_css('aside.right-sidebar.right-sidebar-collapsed')
+      end
+    end
+  end
+
   context 'as a guest' do
     before do
       project.team << [user, :guest]

spec/support/mobile_helpers.rb

 module MobileHelpers
+  def resize_screen_xs
+    resize_window(767, 768)
+  end
+
   def resize_screen_sm
     resize_window(900, 768)
   end