Merge branch 'dm-commit-diff-discussions-in-mr-context' into 'master'

Allow commenting on individual commits inside an MR

Closes #31847

See merge request gitlab-org/gitlab-ce!12148

28 files changed, 236 insertions, 95 deletions

diff --git a/app/assets/javascripts/diff_notes/diff_notes_bundle.js b/app/assets/javascripts/diff_notes/diff_notes_bundle.js
index 0863c3406bd..e0422057090 100644
--- a/app/assets/javascripts/diff_notes/diff_notes_bundle.js
+++ b/app/assets/javascripts/diff_notes/diff_notes_bundle.js
@@ -16,7 +16,8 @@ import './components/diff_note_avatars';
 import './components/new_issue_for_discussion';
 
 $(() => {
-  const projectPath = document.querySelector('.merge-request').dataset.projectPath;
+  const projectPathHolder = document.querySelector('.merge-request') || document.querySelector('.commit-box');
+  const projectPath = projectPathHolder.dataset.projectPath;
   const COMPONENT_SELECTOR = 'resolve-btn, resolve-discussion-btn, jump-to-discussion, comment-and-resolve-btn, new-issue-for-discussion-btn';
 
   window.gl = window.gl || {};
diff --git a/app/assets/javascripts/diff_notes/services/resolve.js b/app/assets/javascripts/diff_notes/services/resolve.js
index 6eae54f830b..96fe23640af 100644
--- a/app/assets/javascripts/diff_notes/services/resolve.js
+++ b/app/assets/javascripts/diff_notes/services/resolve.js
@@ -43,7 +43,7 @@ class ResolveServiceClass {
           discussion.resolveAllNotes(resolvedBy);
         }
 
-        gl.mrWidget.checkStatus();
+        if (gl.mrWidget) gl.mrWidget.checkStatus();
         discussion.updateHeadline(data);
       })
       .catch(() => new Flash('An error occurred when trying to resolve a discussion. Please try again.'));
diff --git a/app/controllers/projects/commit_controller.rb b/app/controllers/projects/commit_controller.rb
index 6ff96a3f295..2e7344b1cad 100644
--- a/app/controllers/projects/commit_controller.rb
+++ b/app/controllers/projects/commit_controller.rb
@@ -134,6 +134,23 @@ class Projects::CommitController < Projects::ApplicationController
     @grouped_diff_discussions = commit.grouped_diff_discussions
     @discussions = commit.discussions
 
+    if merge_request_iid = params[:merge_request_iid]
+      @merge_request = MergeRequestsFinder.new(current_user, project_id: @project.id).find_by(iid: merge_request_iid)
+
+      if @merge_request
+        @new_diff_note_attrs.merge!(
+          noteable_type: 'MergeRequest',
+          noteable_id: @merge_request.id
+        )
+
+        merge_request_commit_notes = @merge_request.notes.where(commit_id: @commit.id).inc_relations_for_view
+        merge_request_commit_diff_discussions = merge_request_commit_notes.grouped_diff_discussions(@commit.diff_refs)
+        @grouped_diff_discussions.merge!(merge_request_commit_diff_discussions) do |line_code, left, right|
+          left + right
+        end
+      end
+    end
+
     @notes = (@grouped_diff_discussions.values.flatten + @discussions).flat_map(&:notes)
     @notes = prepare_notes_for_rendering(@notes, @commit)
   end
diff --git a/app/controllers/projects/merge_requests/application_controller.rb b/app/controllers/projects/merge_requests/application_controller.rb
index 1269759fc2b..793ae03fb88 100644
--- a/app/controllers/projects/merge_requests/application_controller.rb
+++ b/app/controllers/projects/merge_requests/application_controller.rb
@@ -28,7 +28,6 @@ class Projects::MergeRequests::ApplicationController < Projects::ApplicationCont
       :task_num,
       :title,
       :discussion_locked,
-
       label_ids: []
     ]
   end
diff --git a/app/controllers/projects/merge_requests/diffs_controller.rb b/app/controllers/projects/merge_requests/diffs_controller.rb
index 9f966889995..fe8525a488c 100644
--- a/app/controllers/projects/merge_requests/diffs_controller.rb
+++ b/app/controllers/projects/merge_requests/diffs_controller.rb
@@ -4,6 +4,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   include RendersNotes
 
   before_action :apply_diff_view_cookie!
+  before_action :commit
   before_action :define_diff_vars
   before_action :define_diff_comment_vars
 
@@ -20,18 +21,33 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   private
 
   def define_diff_vars
+    @merge_request_diffs = @merge_request.merge_request_diffs.viewable.order_id_desc
+    @compare = commit || find_merge_request_diff_compare
+    return render_404 unless @compare
+
+    @diffs = @compare.diffs(diff_options)
+  end
+
+  def commit
+    return nil unless commit_id = params[:commit_id].presence
+    return nil unless @merge_request.all_commits.exists?(sha: commit_id)
+
+    @commit ||= @project.commit(commit_id)
+  end
+
+  def find_merge_request_diff_compare
     @merge_request_diff =
-      if params[:diff_id]
-        @merge_request.merge_request_diffs.viewable.find(params[:diff_id])
+      if diff_id = params[:diff_id].presence
+        @merge_request.merge_request_diffs.viewable.find_by(id: diff_id)
       else
         @merge_request.merge_request_diff
       end
 
-    @merge_request_diffs = @merge_request.merge_request_diffs.viewable.order_id_desc
+    return unless @merge_request_diff
+
     @comparable_diffs = @merge_request_diffs.select { |diff| diff.id < @merge_request_diff.id }
 
-    if params[:start_sha].present?
-      @start_sha = params[:start_sha]
+    if @start_sha = params[:start_sha].presence
       @start_version = @comparable_diffs.find { |diff| diff.head_commit_sha == @start_sha }
 
       unless @start_version
@@ -40,20 +56,18 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
       end
     end
 
-    @compare =
-      if @start_sha
-        @merge_request_diff.compare_with(@start_sha)
-      else
-        @merge_request_diff
-      end
-
-    @diffs = @compare.diffs(diff_options)
+    if @start_sha
+      @merge_request_diff.compare_with(@start_sha)
+    else
+      @merge_request_diff
+    end
   end
 
   def define_diff_comment_vars
     @new_diff_note_attrs = {
       noteable_type: 'MergeRequest',
-      noteable_id: @merge_request.id
+      noteable_id: @merge_request.id,
+      commit_id: @commit&.id
     }
 
     @diff_notes_disabled = false
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 37acd1c9787..e7b3b73024b 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -7,11 +7,8 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   include IssuableCollections
 
   skip_before_action :merge_request, only: [:index, :bulk_update]
-
   before_action :authorize_update_issuable!, only: [:close, :edit, :update, :remove_wip, :sort]
-
   before_action :set_issuables_index, only: [:index]
-
   before_action :authenticate_user!, only: [:assign_related_issues]
 
   def index
diff --git a/app/helpers/commits_helper.rb b/app/helpers/commits_helper.rb
index f68e2cd3afa..2d304f7eb91 100644
--- a/app/helpers/commits_helper.rb
+++ b/app/helpers/commits_helper.rb
@@ -228,4 +228,12 @@ module CommitsHelper
       [commits, 0]
     end
   end
+
+  def commit_path(project, commit, merge_request: nil)
+    if merge_request&.persisted?
+      diffs_project_merge_request_path(project, merge_request, commit_id: commit.id)
+    else
+      project_commit_path(project, commit)
+    end
+  end
 end
diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index 5b2c58d193d..ce57422f45d 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -101,6 +101,30 @@ module MergeRequestsHelper
     }.merge(merge_params_ee(merge_request))
   end
 
+  def tab_link_for(merge_request, tab, options = {}, &block)
+    data_attrs = {
+      action: tab.to_s,
+      target: "##{tab}",
+      toggle: options.fetch(:force_link, false) ? '' : 'tab'
+    }
+
+    url = case tab
+          when :show
+            data_attrs[:target] = '#notes'
+            method(:project_merge_request_path)
+          when :commits
+            method(:commits_project_merge_request_path)
+          when :pipelines
+            method(:pipelines_project_merge_request_path)
+          when :diffs
+            method(:diffs_project_merge_request_path)
+          else
+            raise "Cannot create tab #{tab}."
+          end
+
+    link_to(url[merge_request.project, merge_request], data: data_attrs, &block)
+  end
+
   def merge_params_ee(merge_request)
     {}
   end
diff --git a/app/models/commit.rb b/app/models/commit.rb
index 6b28d290f99..307e4fcedfe 100644
--- a/app/models/commit.rb
+++ b/app/models/commit.rb
@@ -1,3 +1,4 @@
+# coding: utf-8
 class Commit
   extend ActiveModel::Naming
   extend Gitlab::Cache::RequestCache
@@ -25,7 +26,7 @@ class Commit
   DIFF_HARD_LIMIT_FILES = 1000
   DIFF_HARD_LIMIT_LINES = 50000
 
-  MIN_SHA_LENGTH = 7
+  MIN_SHA_LENGTH = Gitlab::Git::Commit::MIN_SHA_LENGTH
   COMMIT_SHA_PATTERN = /\h{#{MIN_SHA_LENGTH},40}/.freeze
 
   def banzai_render_context(field)
diff --git a/app/models/concerns/discussion_on_diff.rb b/app/models/concerns/discussion_on_diff.rb
index f5cbb3becad..4b4d519f3df 100644
--- a/app/models/concerns/discussion_on_diff.rb
+++ b/app/models/concerns/discussion_on_diff.rb
@@ -32,6 +32,10 @@ module DiscussionOnDiff
     first_note.position.new_path
   end
 
+  def on_merge_request_commit?
+    for_merge_request? && commit_id.present?
+  end
+
   # Returns an array of at most 16 highlighted lines above a diff note
   def truncated_diff_lines(highlight: true)
     lines = highlight ? highlighted_diff_lines : diff_lines
diff --git a/app/models/diff_discussion.rb b/app/models/diff_discussion.rb
index 6eba87da1a1..4a65738214b 100644
--- a/app/models/diff_discussion.rb
+++ b/app/models/diff_discussion.rb
@@ -24,7 +24,11 @@ class DiffDiscussion < Discussion
     return unless for_merge_request?
     return {} if active?
 
-    noteable.version_params_for(position.diff_refs)
+    if on_merge_request_commit?
+      { commit_id: commit_id }
+    else
+      noteable.version_params_for(position.diff_refs)
+    end
   end
 
   def reply_attributes
diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index ae5f138a920..b53d44cda95 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -17,6 +17,7 @@ class DiffNote < Note
   validates :noteable_type, inclusion: { in: NOTEABLE_TYPES }
   validate :positions_complete
   validate :verify_supported
+  validate :diff_refs_match_commit, if: :for_commit?
 
   before_validation :set_original_position, on: :create
   before_validation :update_position, on: :create, if: :on_text?
@@ -135,6 +136,12 @@ class DiffNote < Note
     errors.add(:position, "is invalid")
   end
 
+  def diff_refs_match_commit
+    return if self.original_position.diff_refs == self.commit.diff_refs
+
+    errors.add(:commit_id, 'does not match the diff refs')
+  end
+
   def keep_around_commits
     project.repository.keep_around(self.original_position.base_sha)
     project.repository.keep_around(self.original_position.start_sha)
diff --git a/app/models/discussion.rb b/app/models/discussion.rb
index 437df923d2d..92482a1a875 100644
--- a/app/models/discussion.rb
+++ b/app/models/discussion.rb
@@ -11,6 +11,7 @@ class Discussion
             :author,
 
             :noteable,
+            :commit_id,
             :for_commit?,
             :for_merge_request?,
 
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 949d42f865c..422f138c4ea 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -649,6 +649,7 @@ class MergeRequest < ActiveRecord::Base
       .to_sql
 
     Note.from("(#{union}) #{Note.table_name}")
+      .includes(:noteable)
   end
 
   alias_method :discussion_notes, :related_notes
@@ -925,21 +926,27 @@ class MergeRequest < ActiveRecord::Base
       .order(id: :desc)
   end
 
-  # Note that this could also return SHA from now dangling commits
-  #
-  def all_commit_shas
-    return commit_shas unless persisted?
-
-    diffs_relation = merge_request_diffs
-
+  def all_commits
     # MySQL doesn't support LIMIT in a subquery.
-    diffs_relation = diffs_relation.recent if Gitlab::Database.postgresql?
+    diffs_relation = if Gitlab::Database.postgresql?
+                       merge_request_diffs.recent
+                     else
+                       merge_request_diffs
+                     end
 
     MergeRequestDiffCommit
       .where(merge_request_diff: diffs_relation)
       .limit(10_000)
-      .pluck('sha')
-      .uniq
+  end
+
+  # Note that this could also return SHA from now dangling commits
+  #
+  def all_commit_shas
+    @all_commit_shas ||= begin
+      return commit_shas unless persisted?
+
+      all_commits.pluck(:sha).uniq
+    end
   end
 
   def merge_commit
diff --git a/app/models/note.rb b/app/models/note.rb
index 733bbbc013f..c4c2ab8e67d 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -230,16 +230,18 @@ class Note < ActiveRecord::Base
     for_personal_snippet?
   end
 
+  def commit
+    @commit ||= project.commit(commit_id) if commit_id.present?
+  end
+
   # override to return commits, which are not active record
   def noteable
-    if for_commit?
-      @commit ||= project.commit(commit_id)
-    else
-      super
-    end
-  # Temp fix to prevent app crash
-  # if note commit id doesn't exist
+    return commit if for_commit?
+
+    super
   rescue
+    # Temp fix to prevent app crash
+    # if note commit id doesn't exist
     nil
   end
 
@@ -401,6 +403,10 @@ class Note < ActiveRecord::Base
     noteable_object&.touch
   end
 
+  def banzai_render_context(field)
+    super.merge(noteable: noteable)
+  end
+
   private
 
   def keep_around_commit
diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index 434dda89db0..9f05535d4d4 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -6,7 +6,7 @@ module MergeRequests
       @oldrev, @newrev = oldrev, newrev
       @branch_name = Gitlab::Git.ref_name(ref)
 
-      find_new_commits
+      Gitlab::GitalyClient.allow_n_plus_1_calls(&method(:find_new_commits))
       # Be sure to close outstanding MRs before reloading them to avoid generating an
       # empty diff during a manual merge
       close_merge_requests
diff --git a/app/views/discussions/_discussion.html.haml b/app/views/discussions/_discussion.html.haml
index 0f03163a2e8..205320ed87c 100644
--- a/app/views/discussions/_discussion.html.haml
+++ b/app/views/discussions/_discussion.html.haml
@@ -32,9 +32,17 @@
             - elsif discussion.diff_discussion?
               on
               = conditional_link_to url.present?, url do
-                - unless discussion.active?
-                  an old version of
-                the diff
+                - if discussion.on_merge_request_commit?
+                  - unless discussion.active?
+                    an outdated change in
+                  commit
+
+                  %span.commit-sha= Commit.truncate_sha(discussion.commit_id)
+                - else
+                  - unless discussion.active?
+                    an old version of
+                  the diff
+
 
             = time_ago_with_tooltip(discussion.created_at, placement: "bottom", html_class: "note-created-ago")
           = render "discussions/headline", discussion: discussion
diff --git a/app/views/projects/commit/_commit_box.html.haml b/app/views/projects/commit/_commit_box.html.haml
index 5f607c2ab25..09934c09865 100644
--- a/app/views/projects/commit/_commit_box.html.haml
+++ b/app/views/projects/commit/_commit_box.html.haml
@@ -47,7 +47,7 @@
           %li= link_to s_("DownloadCommit|Email Patches"), project_commit_path(@project, @commit, format: :patch)
         %li= link_to s_("DownloadCommit|Plain Diff"),    project_commit_path(@project, @commit, format: :diff)
 
-.commit-box
+.commit-box{ data: { project_path: project_path(@project) } }
   %h3.commit-title
     = markdown(@commit.title, pipeline: :single_line, author: @commit.author)
   - if @commit.description.present?
@@ -80,3 +80,13 @@
       - if last_pipeline.duration
         in
         = time_interval_in_words last_pipeline.duration
+
+  - if @merge_request
+    .well-segment
+      = icon('info-circle fw')
+
+      This commit is part of merge request
+      = succeed '.' do
+        = link_to @merge_request.to_reference, diffs_project_merge_request_path(@project, @merge_request, commit_id: @commit.id)
+
+      Comments created here will be created in the context of that merge request.
diff --git a/app/views/projects/commit/show.html.haml b/app/views/projects/commit/show.html.haml
index abb292f8f27..2890e9d2b65 100644
--- a/app/views/projects/commit/show.html.haml
+++ b/app/views/projects/commit/show.html.haml
@@ -6,6 +6,9 @@
 - @content_class = limited_container_width
 - page_title        "#{@commit.title} (#{@commit.short_id})", "Commits"
 - page_description  @commit.description
+- content_for :page_specific_javascripts do
+  = page_specific_javascript_bundle_tag('common_vue')
+  = page_specific_javascript_bundle_tag('diff_notes')
 
 .container-fluid{ class: [limited_container_width, container_class] }
   = render "commit_box"
diff --git a/app/views/projects/commits/_commit.html.haml b/app/views/projects/commits/_commit.html.haml
index 1b91a94a9f8..618a6355d23 100644
--- a/app/views/projects/commits/_commit.html.haml
+++ b/app/views/projects/commits/_commit.html.haml
@@ -1,7 +1,18 @@
-- ref = local_assigns.fetch(:ref)
-
-- cache_key = [project.full_path, commit.id, current_application_settings, @path.presence, current_controller?(:commits), I18n.locale]
-- cache_key.push(commit.status(ref)) if commit.status(ref)
+- view_details  = local_assigns.fetch(:view_details, false)
+- merge_request = local_assigns.fetch(:merge_request, nil)
+- project       = local_assigns.fetch(:project) { merge_request&.project }
+- ref           = local_assigns.fetch(:ref) { merge_request&.source_branch }
+
+- link = commit_path(project, commit, merge_request: merge_request)
+- cache_key = [project.full_path,
+               commit.id,
+               current_application_settings,
+               @path.presence,
+               current_controller?(:commits),
+               merge_request&.iid,
+               view_details,
+               commit.status(ref),
+               I18n.locale].compact
 
 = cache(cache_key, expires_in: 1.day) do
   %li.commit.flex-row.js-toggle-container{ id: "commit-#{commit.short_id}" }
@@ -11,7 +22,7 @@
 
     .commit-detail
       .commit-content
-        = link_to_markdown_field(commit, :title, project_commit_path(project, commit.id), class: "commit-row-message item-title")
+        = link_to_markdown_field(commit, :title, link, class: "commit-row-message item-title")
         %span.commit-row-message.visible-xs-inline
           &middot;
           = commit.short_id
@@ -31,8 +42,7 @@
           - commit_text =  _('%{commit_author_link} committed %{commit_timeago}') % { commit_author_link: commit_author_link, commit_timeago: commit_timeago }
           #{ commit_text.html_safe }
 
-
-      .commit-actions.hidden-xs
+      .commit-actions.flex-row.hidden-xs
         - if request.xhr?
           = render partial: 'projects/commit/signature', object: commit.signature
         - else
@@ -41,6 +51,9 @@
         - if commit.status(ref)
           = render_commit_status(commit, ref: ref)
 
-        = link_to commit.short_id, project_commit_path(project, commit), class: "commit-sha btn btn-transparent btn-link"
+        = link_to commit.short_id, link, class: "commit-sha btn btn-transparent btn-link"
         = clipboard_button(text: commit.id, title: _("Copy commit SHA to clipboard"))
         = link_to_browse_code(project, commit)
+
+        - if view_details && merge_request
+          = link_to "View details", project_commit_path(project, commit.id, merge_request_iid: merge_request.iid), class: "btn btn-default"
diff --git a/app/views/projects/commits/_commits.html.haml b/app/views/projects/commits/_commits.html.haml
index d14897428d0..ac6852751be 100644
--- a/app/views/projects/commits/_commits.html.haml
+++ b/app/views/projects/commits/_commits.html.haml
@@ -1,4 +1,7 @@
-- ref = local_assigns.fetch(:ref)
+- merge_request = local_assigns.fetch(:merge_request, nil)
+- project = local_assigns.fetch(:project) { merge_request&.project }
+- ref = local_assigns.fetch(:ref) { merge_request&.source_branch }
+
 - commits, hidden = limited_commits(@commits)
 
 - commits.chunk { |c| c.committed_date.in_time_zone.to_date }.each do |day, commits|
@@ -8,7 +11,7 @@
 
   %li.commits-row{ data: { day: day } }
     %ul.content-list.commit-list.flex-list
-      = render partial: 'projects/commits/commit', collection: commits, locals: { project: project, ref: ref }
+      = render partial: 'projects/commits/commit', collection: commits, locals: { project: project, ref: ref, merge_request: merge_request }
 
 - if hidden > 0
   %li.alert.alert-warning
diff --git a/app/views/projects/merge_requests/_commits.html.haml b/app/views/projects/merge_requests/_commits.html.haml
index 11793919ff7..b414518b597 100644
--- a/app/views/projects/merge_requests/_commits.html.haml
+++ b/app/views/projects/merge_requests/_commits.html.haml
@@ -5,4 +5,4 @@
     = custom_icon ('illustration_no_commits')
 - else
   %ol#commits-list.list-unstyled
-    = render "projects/commits/commits", project: @merge_request.source_project, ref: @merge_request.source_branch
+    = render "projects/commits/commits", merge_request: @merge_request
diff --git a/app/views/projects/merge_requests/diffs/_commit_widget.html.haml b/app/views/projects/merge_requests/diffs/_commit_widget.html.haml
new file mode 100644
index 00000000000..2e5594f8cbe
--- /dev/null
+++ b/app/views/projects/merge_requests/diffs/_commit_widget.html.haml
@@ -0,0 +1,5 @@
+- if @commit
+  .info-well.hidden-xs.prepend-top-default
+    .well-segment
+      %ul.blob-commit-info
+        = render 'projects/commits/commit', commit: @commit, merge_request: @merge_request, view_details: true
diff --git a/app/views/projects/merge_requests/diffs/_different_base.html.haml b/app/views/projects/merge_requests/diffs/_different_base.html.haml
new file mode 100644
index 00000000000..0e57066f9c9
--- /dev/null
+++ b/app/views/projects/merge_requests/diffs/_different_base.html.haml
@@ -0,0 +1,11 @@
+- if @merge_request_diff && different_base?(@start_version, @merge_request_diff)
+  .mr-version-controls
+    .content-block
+      = icon('info-circle')
+      Selected versions have different base commits.
+      Changes will include
+      = link_to project_compare_path(@project, from: @start_version.base_commit_sha, to: @merge_request_diff.base_commit_sha) do
+        new commits
+      from
+      = succeed '.' do
+        %code.ref-name= @merge_request.target_branch
diff --git a/app/views/projects/merge_requests/diffs/_diffs.html.haml b/app/views/projects/merge_requests/diffs/_diffs.html.haml
index 3d7a8f9d870..60c91024b23 100644
--- a/app/views/projects/merge_requests/diffs/_diffs.html.haml
+++ b/app/views/projects/merge_requests/diffs/_diffs.html.haml
@@ -1,13 +1,18 @@
-- if @merge_request_diff.collected? || @merge_request_diff.overflow?
-  = render 'projects/merge_requests/diffs/versions'
-  = render "projects/diffs/diffs", diffs: @diffs, environment: @environment, merge_request: true
-- elsif @merge_request_diff.empty?
+= render 'projects/merge_requests/diffs/version_controls'
+= render 'projects/merge_requests/diffs/different_base'
+= render 'projects/merge_requests/diffs/not_all_comments_displayed'
+= render 'projects/merge_requests/diffs/commit_widget'
+
+- if @merge_request_diff&.empty?
   .nothing-here-block
     = image_tag 'illustrations/merge_request_changes_empty.svg'
-    %p
-      Nothing to merge from
-      %strong= @merge_request.source_branch
-      into
-      %strong= @merge_request.target_branch
-
+    = succeed '.' do
+      No changes between
+      %span.ref-name= @merge_request.source_branch
+      and
+      %span.ref-name= @merge_request.target_branch
       %p= link_to 'Create commit', project_new_blob_path(@project, @merge_request.source_branch), class: 'btn btn-save'
+- else
+  - diff_viewable = @merge_request_diff ? @merge_request_diff.collected? || @merge_request_diff.overflow? : true
+  - if diff_viewable
+    = render "projects/diffs/diffs", diffs: @diffs, environment: @environment, merge_request: true
diff --git a/app/views/projects/merge_requests/diffs/_not_all_comments_displayed.html.haml b/app/views/projects/merge_requests/diffs/_not_all_comments_displayed.html.haml
new file mode 100644
index 00000000000..529fbb8547a
--- /dev/null
+++ b/app/views/projects/merge_requests/diffs/_not_all_comments_displayed.html.haml
@@ -0,0 +1,17 @@
+- if @commit || @start_version || (@merge_request_diff && !@merge_request_diff.latest?)
+  .mr-version-controls
+    .content-block.comments-disabled-notif.clearfix
+      = icon('info-circle')
+      = succeed '.' do
+        - if @commit
+          Only comments from the following commit are shown below
+        - else
+          Not all comments are displayed because you're
+          - if @start_version
+            comparing two versions of the diff
+          - else
+            viewing an old version of the diff
+      .pull-right
+        = link_to diffs_project_merge_request_path(@merge_request.project, @merge_request), class: 'btn btn-sm' do
+          Show latest version
+          = "of the diff" if @commit
diff --git a/app/views/projects/merge_requests/diffs/_versions.html.haml b/app/views/projects/merge_requests/diffs/_version_controls.html.haml
index 9f7152b9824..1c26f0405d2 100644
--- a/app/views/projects/merge_requests/diffs/_versions.html.haml
+++ b/app/views/projects/merge_requests/diffs/_version_controls.html.haml
@@ -1,4 +1,4 @@
-- if @merge_request_diffs.size > 1
+- if @merge_request_diff && @merge_request_diffs.size > 1
   .mr-version-controls
     .mr-version-menus-container.content-block
       Changes between
@@ -71,27 +71,3 @@
                         (base)
                     %div
                       %strong.commit-sha= short_sha(@merge_request_diff.base_commit_sha)
-
-    - if different_base?(@start_version, @merge_request_diff)
-      .content-block
-        = icon('info-circle')
-        Selected versions have different base commits.
-        Changes will include
-        = link_to project_compare_path(@project, from: @start_version.base_commit_sha, to: @merge_request_diff.base_commit_sha) do
-          new commits
-        from
-        = succeed '.' do
-          %code= @merge_request.target_branch
-
-    - if @start_version || !@merge_request_diff.latest?
-      .comments-disabled-notif.content-block
-        = icon('info-circle')
-        Not all comments are displayed because you're
-        - if @start_version
-          comparing two versions
-        - else
-          viewing an old version
-        of the diff.
-
-        .pull-right
-          = link_to 'Show latest version', diffs_project_merge_request_path(@project, @merge_request), class: 'btn btn-sm'
diff --git a/app/views/projects/merge_requests/show.html.haml b/app/views/projects/merge_requests/show.html.haml
index d88e3d794d3..abff702fd9d 100644
--- a/app/views/projects/merge_requests/show.html.haml
+++ b/app/views/projects/merge_requests/show.html.haml
@@ -8,7 +8,7 @@
   = webpack_bundle_tag('common_vue')
   = webpack_bundle_tag('diff_notes')
 
-.merge-request{ 'data-mr-action': "#{j params[:tab].presence || 'show'}", 'data-url' => merge_request_path(@merge_request, format: :json), 'data-project-path' => project_path(@merge_request.project) }
+.merge-request{ data: { mr_action: j(params[:tab].presence || 'show'), url: merge_request_path(@merge_request, format: :json), project_path: project_path(@merge_request.project) } }
   = render "projects/merge_requests/mr_title"
 
   .merge-request-details.issuable-details{ data: { id: @merge_request.project.id } }
@@ -38,21 +38,21 @@
           .nav-links.scrolling-tabs
             %ul.merge-request-tabs
               %li.notes-tab
-                = link_to project_merge_request_path(@project, @merge_request), data: { target: 'div#notes', action: 'show', toggle: 'tab' } do
+                = tab_link_for @merge_request, :show, force_link: @commit.present? do
                   Discussion
                   %span.badge= @merge_request.related_notes.user.count
               - if @merge_request.source_project
                 %li.commits-tab
-                  = link_to commits_project_merge_request_path(@project, @merge_request), data: { target: 'div#commits', action: 'commits', toggle: 'tab' } do
+                  = tab_link_for @merge_request, :commits do
                     Commits
                     %span.badge= @commits_count
               - if @pipelines.any?
                 %li.pipelines-tab
-                  = link_to pipelines_project_merge_request_path(@project, @merge_request), data: { target: '#pipelines', action: 'pipelines', toggle: 'tab' } do
+                  = tab_link_for @merge_request, :pipelines do
                     Pipelines
                     %span.badge.js-pipelines-mr-count= @pipelines.size
               %li.diffs-tab
-                = link_to diffs_project_merge_request_path(@project, @merge_request), data: { target: 'div#diffs', action: 'diffs', toggle: 'tab' } do
+                = tab_link_for @merge_request, :diffs do
                   Changes
                   %span.badge= @merge_request.diff_size
         #resolve-count-app.line-resolve-all-container.prepend-top-10{ "v-cloak" => true }