Trigger autosize update after template selection

Closes #21982

CHANGELOG

@@ -27,6 +27,7 @@ v 8.12.0 (unreleased)
   - Fix bug where pagination is still displayed despite all todos marked as done (ClemMakesApps)
   - Center build stage columns in pipeline overview (ClemMakesApps)
   - Rename behaviour to behavior in bug issue template for consistency (ClemMakesApps)
+  - Fix bug stopping issue description being scrollable after selecting issue template
   - Remove suggested colors hover underline (ClemMakesApps)
   - Shorten task status phrase (ClemMakesApps)
   - Fix project visibility level fields on settings

app/assets/javascripts/blob/template_selector.js

@@ -13,6 +13,9 @@
       this.buildDropdown();
       this.bindEvents();
       this.onFilenameUpdate();
+
+      this.autosizeUpdateEvent = document.createEvent('Event');
+      this.autosizeUpdateEvent.initEvent('autosize:update', true, false);
     }
 
     TemplateSelector.prototype.buildDropdown = function() {
@@ -69,6 +72,10 @@
     TemplateSelector.prototype.requestFileSuccess = function(file, skipFocus) {
       this.editor.setValue(file.content, 1);
       if (!skipFocus) this.editor.focus();
+
+      if (this.editor instanceof jQuery) {
+        this.editor.get(0).dispatchEvent(this.autosizeUpdateEvent);
+      }
     };
 
     TemplateSelector.prototype.startLoadingSpinner = function() {

spec/features/projects/issuable_templates_spec.rb

@@ -13,10 +13,12 @@ feature 'issuable templates', feature: true, js: true do
 
   context 'user creates an issue using templates' do
     let(:template_content) { 'this is a test "bug" template' }
+    let(:longtemplate_content) { %Q(this\n\n\n\n\nis\n\n\n\n\na\n\n\n\n\nbug\n\n\n\n\ntemplate) }
     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)
+      project.repository.commit_file(user, '.gitlab/issue_templates/test.md', longtemplate_content, 'added issue template', 'master', false)
       visit edit_namespace_project_issue_path project.namespace, project, issue
       fill_in :'issue[title]', with: 'test issue title'
     end
@@ -27,6 +29,17 @@ feature 'issuable templates', feature: true, js: true do
       preview_template
       save_changes
     end
+
+    it 'updates height of markdown textarea' do
+      start_height = page.evaluate_script('$(".markdown-area").outerHeight()')
+
+      select_template 'test'
+      wait_for_ajax
+
+      end_height = page.evaluate_script('$(".markdown-area").outerHeight()')
+      
+      expect(end_height).not_to eq(start_height)
+    end
   end
 
   context 'user creates a merge request using templates' do