Fix Merge Request "Edit in Web IDE" dropdown link on MR diffs page

app/helpers/blob_helper.rb

@@ -26,6 +26,25 @@ module BlobHelper
     File.join(segments)
   end
 
+  def ide_merge_request_path(merge_request, path = '')
+    target_project = merge_request.target_project
+    source_project = merge_request.source_project
+
+    if merge_request.merged?
+      branch = merge_request.target_branch_exists? ? merge_request.target_branch : target_project.default_branch
+
+      return ide_edit_path(target_project, branch, path)
+    end
+
+    if target_project != source_project
+      params = { target_project: target_project.full_path }
+    end
+
+    result = File.join(ide_path, 'project', source_project.full_path, 'merge_requests', merge_request.to_param)
+    result += "?#{params.to_query}" unless params.nil?
+    result
+  end
+
   def ide_fork_and_edit_path(project = @project, ref = @ref, path = @path, options = {})
     fork_path_for_current_user(project, ide_edit_path(project, ref, path))
   end

app/helpers/gitlab_routing_helper.rb

@@ -343,18 +343,6 @@ module GitlabRoutingHelper
     Gitlab::UrlBuilder.wiki_page_url(wiki, page, only_path: true, **options)
   end
 
-  def gitlab_ide_merge_request_path(merge_request)
-    target_project = merge_request.target_project
-    source_project = merge_request.source_project
-    params = {}
-
-    if target_project != source_project
-      params = { target_project: target_project.full_path }
-    end
-
-    ide_merge_request_path(source_project.namespace, source_project, merge_request, params)
-  end
-
   private
 
   def snippet_query_params(snippet, *args)

app/serializers/diff_file_base_entity.rb

@@ -48,7 +48,7 @@ class DiffFileBaseEntity < Grape::Entity
 
     next unless has_edit_path?(merge_request)
 
-    gitlab_ide_merge_request_path(merge_request)
+    ide_merge_request_path(merge_request, diff_file.new_path)
   end
 
   expose :old_path_html do |diff_file|

changelogs/unreleased/268301-mr-ide-button.yml

+---
+title: Fix Merge Request "Edit in Web IDE" dropdown link on MR diffs page
+merge_request: 45653
+author:
+type: fixed

config/routes.rb

@@ -122,7 +122,6 @@ Rails.application.routes.draw do
 
     get 'ide' => 'ide#index'
     get 'ide/*vueroute' => 'ide#index', format: false
-    get 'ide/project/:namespace/:project/merge_requests/:id' => 'ide#index', format: false, as: :ide_merge_request
 
     draw :operations
     draw :jira_connect

spec/helpers/blob_helper_spec.rb

@@ -444,6 +444,55 @@ RSpec.describe BlobHelper do
     end
   end
 
+  describe '#ide_merge_request_path' do
+    let_it_be(:project) { create(:project, :repository) }
+    let_it_be(:merge_request) { create(:merge_request, source_project: project)}
+
+    it 'returns IDE path for the given MR if MR is not merged' do
+      expect(helper.ide_merge_request_path(merge_request)).to eq("/-/ide/project/#{project.full_path}/merge_requests/#{merge_request.iid}")
+    end
+
+    context 'when the MR comes from a fork' do
+      include ProjectForksHelper
+
+      let(:forked_project) { fork_project(project, nil, repository: true) }
+      let(:merge_request) { create(:merge_request, source_project: forked_project, target_project: project) }
+
+      it 'returns IDE path for MR in the forked repo with target project included as param' do
+        expect(helper.ide_merge_request_path(merge_request)).to eq("/-/ide/project/#{forked_project.full_path}/merge_requests/#{merge_request.iid}?target_project=#{CGI.escape(project.full_path)}")
+      end
+    end
+
+    context 'when the MR is merged' do
+      let(:current_user) { build(:user) }
+
+      let_it_be(:merge_request) { create(:merge_request, :merged, source_project: project, source_branch: 'testing-1', target_branch: 'feature-1') }
+
+      before do
+        allow(helper).to receive(:current_user).and_return(current_user)
+        allow(helper).to receive(:can?).and_return(true)
+      end
+
+      it 'returns default IDE url with master branch' do
+        expect(helper.ide_merge_request_path(merge_request)).to eq("/-/ide/project/#{project.full_path}/edit/master")
+      end
+
+      it 'includes file path passed' do
+        expect(helper.ide_merge_request_path(merge_request, 'README.md')).to eq("/-/ide/project/#{project.full_path}/edit/master/-/README.md")
+      end
+
+      context 'when target branch exists' do
+        before do
+          allow(merge_request).to receive(:target_branch_exists?).and_return(true)
+        end
+
+        it 'returns IDE edit url with the target branch' do
+          expect(helper.ide_merge_request_path(merge_request)).to eq("/-/ide/project/#{project.full_path}/edit/feature-1")
+        end
+      end
+    end
+  end
+
   describe '#ide_fork_and_edit_path' do
     let_it_be(:project) { create(:project) }
     let_it_be(:user) { create(:user) }