Fix bug when decrementing open MR count

We have 2 js-merge-counter elements so calling `text()` returns the
combined text of the 2 elements. This results in a bug when parsing the
number.

Changelog: fixed

app/assets/javascripts/merge_request.js

@@ -124,7 +124,7 @@ MergeRequest.prototype.submitNoteForm = function (form, $button) {
 
 MergeRequest.decreaseCounter = function (by = 1) {
   const $el = $('.js-merge-counter');
-  const count = Math.max(parseInt($el.text().replace(/[^\d]/, ''), 10) - by, 0);
+  const count = Math.max(parseInt($el.first().text().replace(/[^\d]/, ''), 10) - by, 0);
 
   $el.text(addDelimiter(count));
 };

spec/features/merge_request/user_merges_merge_request_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe "User merges a merge request", :js do
   end
 
   shared_examples "fast forward merge a merge request" do
-    it "merges a merge request", :sidekiq_might_not_need_inline do
+    it "merges a merge request", :sidekiq_inline do
       expect(page).to have_content("Fast-forward merge without a merge commit").and have_button("Merge")
 
       page.within(".mr-state-widget") do
@@ -42,4 +42,23 @@ RSpec.describe "User merges a merge request", :js do
       it_behaves_like "fast forward merge a merge request"
     end
   end
+
+  context 'sidebar merge requests counter' do
+    let(:project) { create(:project, :public, :repository) }
+    let!(:merge_request) { create(:merge_request, source_project: project) }
+
+    it 'decrements the open MR count', :sidekiq_inline do
+      create(:merge_request, source_project: project, source_branch: 'branch-1')
+
+      visit(merge_request_path(merge_request))
+
+      expect(page).to have_css('.js-merge-counter', text: '2')
+
+      page.within(".mr-state-widget") do
+        click_button("Merge")
+      end
+
+      expect(page).to have_css('.js-merge-counter', text: '1')
+    end
+  end
 end