Add 'previously merged commits' compare dropdown

app/assets/javascripts/diffs/components/compare_dropdown_layout.vue

 <script>
-import { GlDropdown, GlDropdownItem } from '@gitlab/ui';
+import { GlDropdown, GlDropdownItem, GlDropdownDivider } from '@gitlab/ui';
 import TimeAgo from '~/vue_shared/components/time_ago_tooltip.vue';
 
 export default {
   components: {
     GlDropdown,
     GlDropdownItem,
+    GlDropdownDivider,
     TimeAgo,
   },
   props: {
@@ -24,8 +25,9 @@ export default {
 
 <template>
   <gl-dropdown :text="selectedVersionName" data-qa-selector="dropdown_content">
+    <template v-for="version in versions">
+      <gl-dropdown-divider v-if="version.addDivider" :key="version.id" />
       <gl-dropdown-item
-      v-for="version in versions"
         :key="version.id"
         :class="{
           'is-active': version.selected,
@@ -53,5 +55,6 @@ export default {
           </small>
         </div>
       </gl-dropdown-item>
+    </template>
   </gl-dropdown>
 </template>

app/assets/javascripts/diffs/store/getters_versions_dropdowns.js

@@ -7,6 +7,10 @@ export const selectedTargetIndex = (state) =>
 
 export const selectedSourceIndex = (state) => state.mergeRequestDiff.version_index;
 
+
+export const selectedContextCommitsDiffs = (state) =>
+  state.contextCommitsDiff && state.contextCommitsDiff.showing_context_commits_diff;
+
 export const diffCompareDropdownTargetVersions = (state, getters) => {
   // startVersion only exists if the user has selected a version other
   // than "base" so if startVersion is null then base must be selected
@@ -58,7 +62,7 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
 
 export const diffCompareDropdownSourceVersions = (state, getters) => {
   // Appended properties here are to make the compare_dropdown_layout easier to reason about
-  return state.mergeRequestDiffs.map((v, i) => {
+  const versions = state.mergeRequestDiffs.map((v, i) => {
     const isLatestVersion = i === 0;
 
     return {
@@ -69,7 +73,19 @@ export const diffCompareDropdownSourceVersions = (state, getters) => {
       versionName: isLatestVersion
         ? __('latest version')
         : sprintf(__(`version %{versionIndex}`), { versionIndex: v.version_index }),
-      selected: v.version_index === getters.selectedSourceIndex,
+      selected: v.version_index === getters.selectedSourceIndex && !getters.selectedContextCommitsDiffs,
     };
   });
+
+  const { contextCommitsDiff } = state;
+  if (contextCommitsDiff) {
+    versions.push({
+      href: contextCommitsDiff.diffs_path,
+      commitsText: n__(`%d commit`, `%d commits`, contextCommitsDiff.commits_count),
+      versionName: __('previously merged commits'),
+      selected: getters.selectedContextCommitsDiffs,
+      addDivider: state.mergeRequestDiffs.length > 0,
+    });
+  }
+  return versions;
 };

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -47,7 +47,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
     diffs = @compare.diffs(diff_options)
 
     render json: DiffsMetadataSerializer.new(project: @merge_request.project, current_user: current_user)
-                   .represent(diffs, additional_attributes)
+                   .represent(diffs, additional_attributes.merge(only_context_commits: show_only_context_commits?))
   end
 
   private
@@ -92,7 +92,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   # rubocop: disable CodeReuse/ActiveRecord
   def commit
     return unless commit_id = params[:commit_id].presence
-    return unless @merge_request.all_commits.exists?(sha: commit_id)
+    return unless @merge_request.all_commits.exists?(sha: commit_id) || @merge_request.recent_context_commits.map(&:id).include?(commit_id)
 
     @commit ||= @project.commit(commit_id)
   end
@@ -122,6 +122,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
       end
     end
 
+    return @merge_request.context_commits_diff if show_only_context_commits? && !@merge_request.context_commits_diff.empty?
     return @merge_request.merge_head_diff if render_merge_ref_head_diff?
 
     if @start_sha

app/controllers/projects/merge_requests_controller.rb

@@ -114,6 +114,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
         @noteable = @merge_request
         @commits_count = @merge_request.commits_count + @merge_request.context_commits_count
+        @diffs_count = get_diffs_count
         @issuable_sidebar = serializer.represent(@merge_request, serializer: 'sidebar')
         @current_user_data = UserSerializer.new(project: @project).represent(current_user, {}, MergeRequestCurrentUserEntity).to_json
         @show_whitespace_default = current_user.nil? || current_user.show_whitespace_in_diffs
@@ -386,6 +387,14 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
   private
 
+  def get_diffs_count
+    if show_only_context_commits?
+      @merge_request.context_commits_diff.raw_diffs.size
+    else
+      @merge_request.diff_size
+    end
+  end
+
   def merge_request_update_params
     merge_request_params.merge!(params.permit(:merge_request_diff_head_sha))
   end

app/helpers/diff_helper.rb

@@ -23,14 +23,16 @@ module DiffHelper
     end
   end
 
+  def show_only_context_commits?
+    !!params[:only_context_commits] || @merge_request&.commits&.empty?
+  end
+
   def diff_options
     options = { ignore_whitespace_change: hide_whitespace?, expanded: diffs_expanded? }
 
     if action_name == 'diff_for_path'
       options[:expanded] = true
       options[:paths] = params.values_at(:old_path, :new_path)
-    elsif action_name == 'show'
-      options[:include_context_commits] = true unless @project.context_commits_enabled?
     end
 
     options

app/models/context_commits_diff.rb

+# frozen_string_literal: true
+
+class ContextCommitsDiff
+  include ActsAsPaginatedDiff
+
+  attr_reader :merge_request
+
+  def initialize(merge_request)
+    @merge_request = merge_request
+  end
+
+  def empty?
+    commits.empty?
+  end
+
+  def commits_count
+    merge_request.context_commits_count
+  end
+
+  def diffs(diff_options = nil)
+    Gitlab::Diff::FileCollection::Compare.new(
+      self,
+      project: merge_request.project,
+      diff_options: diff_options,
+      diff_refs: diff_refs
+    )
+  end
+
+  def raw_diffs(options = {})
+    compare.diffs(options.merge(paths: paths))
+  end
+
+  def diff_refs
+    Gitlab::Diff::DiffRefs.new(
+      base_sha: commits.last.diff_refs.base_sha,
+      head_sha: commits.first.diff_refs.head_sha
+    )
+  end
+
+  private
+
+  def compare
+    @compare ||=
+      Gitlab::Git::Compare.new(
+        merge_request.project.repository.raw_repository,
+        commits.last.diff_refs.base_sha,
+        commits.first.diff_refs.head_sha
+      )
+  end
+
+  def commits
+    @commits ||= merge_request.project.repository.commits_by(oids: merge_request.recent_context_commits.map(&:id))
+  end
+
+  def paths
+    merge_request.merge_request_context_commit_diff_files.map(&:path)
+  end
+end
\ No newline at end of file

app/models/merge_request.rb

@@ -1899,6 +1899,11 @@ class MergeRequest < ApplicationRecord
     diff_stats.map(&:path).include?(project.ci_config_path_or_default)
   end
 
+  def context_commits_diff
+    strong_memoize(:context_commits_diff) do
+      ContextCommitsDiff.new(self)
+    end
+  end
   private
 
   def missing_report_error(report_type)

app/models/merge_request_context_commit_diff_file.rb

@@ -16,4 +16,8 @@ class MergeRequestContextCommitDiffFile < ApplicationRecord
   def self.bulk_insert(*args)
     Gitlab::Database.bulk_insert('merge_request_context_commit_diff_files', *args) # rubocop:disable Gitlab/BulkInsert
   end
+
+  def path
+    new_path.presence || old_path
+  end
 end

app/models/merge_request_diff.rb

@@ -665,10 +665,6 @@ class MergeRequestDiff < ApplicationRecord
     opening_external_diff do
       collection = merge_request_diff_files
 
-      if options[:include_context_commits]
-        collection += merge_request.merge_request_context_commit_diff_files
-      end
-
       if paths = options[:paths]
         collection = collection.where('old_path IN (?) OR new_path IN (?)', paths, paths)
       end

app/serializers/context_commits_diff_entity.rb

+# frozen_string_literal: true
+
+class ContextCommitsDiffEntity < Grape::Entity
+  include Gitlab::Routing
+
+  expose :commits_count
+
+  expose :showing_context_commits_diff do |_,  options|
+    options[:only_context_commits]
+  end
+
+  expose :diffs_path do |diff|
+    merge_request = diff.merge_request
+    project = merge_request.target_project
+
+    next unless project
+
+    diffs_project_merge_request_path(project, merge_request, only_context_commits: true)
+  end
+end
\ No newline at end of file

app/serializers/diffs_entity.rb

@@ -84,6 +84,15 @@ class DiffsEntity < Grape::Entity
     project_blob_path(merge_request.project, merge_request.diff_head_sha)
   end
 
+  expose :context_commits_diff, if: -> (_) { merge_request&.project&.context_commits_enabled? } do |diffs, options|
+    next unless merge_request.context_commits_diff.commits_count > 0
+
+    ContextCommitsDiffEntity.represent(
+      merge_request.context_commits_diff,
+      options
+    )
+  end
+
   def merge_request
     options[:merge_request]
   end

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

@@ -40,7 +40,7 @@
           = render "projects/merge_requests/tabs/tab", name: "diffs", class: "diffs-tab", id: "diffs-tab", qa_selector: "diffs_tab" do
             = tab_link_for @merge_request, :diffs do
               = _("Changes")
-              %span.badge.badge-pill.gl-badge.badge-muted.sm= @merge_request.diff_size
+              %span.badge.badge-pill.gl-badge.badge-muted.sm= @diffs_count
         .d-flex.flex-wrap.align-items-center.justify-content-lg-end
           #js-vue-discussion-counter