Preserve selected versions when comparing MR changes

app/controllers/projects/merge_requests_controller.rb

@@ -98,7 +98,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       @start_version = @comparable_diffs.find { |diff| diff.head_commit_sha == @start_sha }
 
       unless @start_version
-        render_404
+        @start_sha = @merge_request_diff.head_commit_sha
+        @start_version = @merge_request_diff
       end
     end
 

app/views/projects/merge_requests/show/_versions.html.haml

@@ -19,7 +19,7 @@
             %ul
               - @merge_request_diffs.each do |merge_request_diff|
                 %li
-                  = link_to merge_request_version_path(@project, @merge_request, merge_request_diff), class: ('is-active' if merge_request_diff == @merge_request_diff) do
+                  = link_to merge_request_version_path(@project, @merge_request, merge_request_diff, @start_sha), class: ('is-active' if merge_request_diff == @merge_request_diff) do
                     %strong
                       - if merge_request_diff.latest?
                         latest version

spec/features/merge_requests/merge_request_versions_spec.rb

@@ -81,4 +81,52 @@ feature 'Merge Request versions', js: true, feature: true do
       expect(page).to have_content '8 changed files'
     end
   end
+
+  describe 'compare with same version' do
+    before do
+      page.within '.mr-version-compare-dropdown' do
+        find('.btn-default').click
+        click_link 'version 1'
+      end
+    end
+
+    it 'should have 0 chages between versions' do
+      page.within '.mr-version-compare-dropdown' do
+        expect(page).to have_content 'version 1'
+      end
+
+      page.within '.mr-version-dropdown' do
+        find('.btn-default').click
+        find(:link, 'version 1').trigger('click')
+      end
+
+      expect(page).to have_content '0 changed files'
+    end
+  end
+
+  describe 'compare with newer version' do
+    before do
+      page.within '.mr-version-compare-dropdown' do
+        find('.btn-default').click
+        click_link 'version 2'
+      end
+    end
+
+    it 'should set the compared versions to be the same' do
+      page.within '.mr-version-compare-dropdown' do
+        expect(page).to have_content 'version 2'
+      end
+
+      page.within '.mr-version-dropdown' do
+        find('.btn-default').click
+        find(:link, 'version 1').trigger('click')
+      end
+
+      page.within '.mr-version-compare-dropdown' do
+        expect(page).to have_content 'version 1'
+      end
+
+      expect(page).to have_content '0 changed files'
+    end
+  end
 end