Add previous revision link

app/assets/stylesheets/framework/files.scss

@@ -207,6 +207,14 @@
           border-left-color: mix($blame-gray, $blame-cyan, $i / 4 * 100%);
         }
       }
+
+      .doc-versions {
+        color: $gray-600;
+
+        &:hover {
+          color: $gray-900;
+        }
+      }
     }
 
     &.logs {

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

@@ -20,8 +20,14 @@
                 .commit-row-title
                   %span.item-title.str-truncated-100
                     = link_to commit.title, project_commit_path(@project, commit.id), class: "cdark", title: commit.title
-                  .float-right
-                    = link_to commit.short_id, project_commit_path(@project, commit), class: "commit-sha"
+                  %span
+                    - previous_commit_id = commit.parent_id
+                    - if previous_commit_id
+                      = link_to project_blame_path(@project, tree_join(previous_commit_id, @path)),
+                                title: _('View blame prior to this change'),
+                                aria: { label: _('View blame prior to this change') },
+                                data: { toggle: 'tooltip', placement: 'right', container: 'body' } do
+                        = sprite_icon('doc-versions', size: 16, css_class: 'doc-versions align-text-bottom')
                    
                 .light
                   = commit_author_link(commit, avatar: false)

changelogs/unreleased/19299-blame-previous-revision.yml

+---
+title: Add previous revision link to blame
+merge_request: 17088
+author: Hiroyuki Sato
+type: added

doc/user/project/repository/git_blame.md

@@ -23,6 +23,11 @@ noted information:
 If you hover over a commit in the UI, you'll see a precise date and time
 for that commit.
 
+![Blame previous commit](img/file_blame_previous_commit_v12_7.png "Blame previous commit")
+
+To see earlier revisions of a specific line, click **View blame prior to this change**
+until you've found the changes you're interested in viewing.
+
 ## Associated `git` command
 
 If you're running `git` from the command line, the equivalent command is

locale/gitlab.pot

@@ -19982,6 +19982,9 @@ msgstr ""
 msgid "View app"
 msgstr ""
 
+msgid "View blame prior to this change"
+msgstr ""
+
 msgid "View dependency details for your project"
 msgstr ""
 

spec/features/projects/files/user_browses_files_spec.rb

@@ -229,6 +229,16 @@ describe "User browses files" do
       expect(page).to have_content("*.rb")
                  .and have_content("Dmitriy Zaporozhets")
                  .and have_content("Initial commit")
+                 .and have_content("Ignore DS files")
+
+      previous_commit_anchor = "//a[@title='Ignore DS files']/parent::span/following-sibling::span/a"
+      find(:xpath, previous_commit_anchor).click
+
+      expect(page).to have_content("*.rb")
+                 .and have_content("Dmitriy Zaporozhets")
+                 .and have_content("Initial commit")
+
+      expect(page).not_to have_content("Ignore DS files")
     end
   end