Disallow editing target branch after merge request is merged

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/223235

app/views/shared/issuable/form/_branch_chooser.html.haml

@@ -17,7 +17,9 @@
       = link_to _('Change branches'), mr_change_branches_path(issuable)
     - elsif issuable.for_fork?
       %code= issuable.target_project_path + ":"
-  - unless issuable.new_record?
+    - if issuable.merged?
+      %code= target_title
+  - unless issuable.new_record? || issuable.merged?
     %span.dropdown.gl-ml-2.d-inline-block
       = form.hidden_field(:target_branch,
         { class: 'target_branch js-target-branch-select ref-name mw-xl',

spec/features/merge_request/user_edits_merge_request_spec.rb

@@ -85,13 +85,24 @@ RSpec.describe 'User edits a merge request', :js do
     end
   end
 
-  it 'changes the target branch' do
-    expect(page).to have_content('From master into feature')
+  describe 'changing target branch' do
+    it 'allows user to change target branch' do
+      expect(page).to have_content('From master into feature')
 
-    select2('merge-test', from: '#merge_request_target_branch')
-    click_button('Save changes')
+      select2('merge-test', from: '#merge_request_target_branch')
+      click_button('Save changes')
+
+      expect(page).to have_content("Request to merge #{merge_request.source_branch} into merge-test")
+      expect(page).to have_content("changed target branch from #{merge_request.target_branch} to merge-test")
+    end
 
-    expect(page).to have_content("Request to merge #{merge_request.source_branch} into merge-test")
-    expect(page).to have_content("changed target branch from #{merge_request.target_branch} to merge-test")
+    describe 'merged merge request' do
+      let(:merge_request) { create(:merge_request, source_project: project, target_project: project, state: :merged) }
+
+      it 'does not allow user to change target branch' do
+        expect(page).to have_content('From master into feature')
+        expect(page).not_to have_selector('.select2-container')
+      end
+    end
   end
 end