Append issue template to existing description

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21733

Add two newlines before the template if the existing description isn't
empty. This makes it easier to see where the template begins.

Don't append the template when "Reset template" is selected, of course.

Don't append template if it equals the existing description. This makes
it so that selecting a template twice doesn't duplicate it.

CHANGELOG

@@ -27,6 +27,7 @@ v 8.13.0 (unreleased)
   - Only update issuable labels if they have been changed
   - Take filters in account in issuable counters. !6496
   - Use custom Ruby images to test builds (registry.dev.gitlab.org/gitlab/gitlab-build-images:*)
+  - Append issue template to existing description !6149 (Joseph Frazier)
   - Revoke button in Applications Settings underlines on hover.
   - Add missing values to linter !6276 (Katarzyna Kobierska Ula Budziszewska)
   - Fix Long commit messages overflow viewport in file tree

app/assets/javascripts/blob/template_selector.js

@@ -72,8 +72,13 @@
     // To be implemented on the extending class
     // e.g.
     // Api.gitignoreText item.name, @requestFileSuccess.bind(@)
-    TemplateSelector.prototype.requestFileSuccess = function(file, skipFocus) {
-      this.editor.setValue(file.content, 1);
+    TemplateSelector.prototype.requestFileSuccess = function(file, skipFocus, append) {
+      var oldValue = this.editor.getValue();
+      var newValue = file.content;
+      if (append && oldValue.length && oldValue !== newValue) {
+        newValue = oldValue + '\n\n' + newValue;
+      }
+      this.editor.setValue(newValue, 1);
       if (!skipFocus) this.editor.focus();
 
       if (this.editor instanceof jQuery) {

app/assets/javascripts/templates/issuable_template_selector.js.es6

@@ -16,7 +16,7 @@
       if (initialQuery.name) this.requestFile(initialQuery);
 
       $('.reset-template', this.dropdown.parent()).on('click', () => {
-        if (this.currentTemplate) this.setInputValueToTemplateContent();
+        if (this.currentTemplate) this.setInputValueToTemplateContent(false);
       });
     }
 
@@ -26,22 +26,24 @@
         this.currentTemplate = currentTemplate;
         if (err) return; // Error handled by global AJAX error handler
         this.stopLoadingSpinner();
-        this.setInputValueToTemplateContent();
+        this.setInputValueToTemplateContent(true);
       });
       return;
     }
 
-    setInputValueToTemplateContent() {
+    setInputValueToTemplateContent(append) {
       // `this.requestFileSuccess` sets the value of the description input field
-      // to the content of the template selected.
+      // to the content of the template selected. If `append` is true, the
+      // template content will be appended to the previous value of the field,
+      // separated by a blank line if the previous value is non-empty.
       if (this.titleInput.val() === '') {
         // If the title has not yet been set, focus the title input and
         // skip focusing the description input by setting `true` as the 2nd
         // argument to `requestFileSuccess`.
-        this.requestFileSuccess(this.currentTemplate, true);
+        this.requestFileSuccess(this.currentTemplate, true, append);
         this.titleInput.focus();
       } else {
-        this.requestFileSuccess(this.currentTemplate);
+        this.requestFileSuccess(this.currentTemplate, false, append);
       }
       return;
     }

spec/features/projects/issuable_templates_spec.rb

@@ -26,7 +26,7 @@ feature 'issuable templates', feature: true, js: true do
     scenario 'user selects "bug" template' do
       select_template 'bug'
       wait_for_ajax
-      preview_template
+      preview_template(template_content)
       save_changes
     end
 
@@ -42,6 +42,26 @@ feature 'issuable templates', feature: true, js: true do
     end
   end
 
+  context 'user creates an issue using templates, with a prior description' do
+    let(:prior_description) { 'test issue description' }
+    let(:template_content) { 'this is a test "bug" template' }
+    let(:issue) { create(:issue, author: user, assignee: user, project: project) }
+
+    background do
+      project.repository.commit_file(user, '.gitlab/issue_templates/bug.md', template_content, 'added issue template', 'master', false)
+      visit edit_namespace_project_issue_path project.namespace, project, issue
+      fill_in :'issue[title]', with: 'test issue title'
+      fill_in :'issue[description]', with: prior_description
+    end
+
+    scenario 'user selects "bug" template' do
+      select_template 'bug'
+      wait_for_ajax
+      preview_template("#{prior_description}\n\n#{template_content}")
+      save_changes
+    end
+  end
+
   context 'user creates a merge request using templates' do
     let(:template_content) { 'this is a test "feature-proposal" template' }
     let(:merge_request) { create(:merge_request, :with_diffs, source_project: project) }
@@ -55,7 +75,7 @@ feature 'issuable templates', feature: true, js: true do
     scenario 'user selects "feature-proposal" template' do
       select_template 'feature-proposal'
       wait_for_ajax
-      preview_template
+      preview_template(template_content)
       save_changes
     end
   end
@@ -82,16 +102,16 @@ feature 'issuable templates', feature: true, js: true do
         scenario 'user selects template' do
           select_template 'feature-proposal'
           wait_for_ajax
-          preview_template
+          preview_template(template_content)
           save_changes
         end
       end
     end
   end
 
-  def preview_template
+  def preview_template(expected_content)
     click_link 'Preview'
-    expect(page).to have_content template_content
+    expect(page).to have_content expected_content
   end
 
   def save_changes