Allow GFM autocomplete to be trigger without the preceding space

Closes #19975

app/assets/javascripts/gfm_auto_complete.js.coffee

@@ -84,6 +84,7 @@ GitLab.GfmAutoComplete =
           @Loading.template
       insertTpl: ':${name}:'
       data: ['loading']
+      startWithSpace: false
       callbacks:
         sorter: @DefaultOptions.sorter
         filter: @DefaultOptions.filter
@@ -100,6 +101,7 @@ GitLab.GfmAutoComplete =
       insertTpl: '${atwho-at}${username}'
       searchKey: 'search'
       data: ['loading']
+      startWithSpace: false
       callbacks:
         sorter: @DefaultOptions.sorter
         filter: @DefaultOptions.filter
@@ -126,6 +128,7 @@ GitLab.GfmAutoComplete =
           @Loading.template
       data: ['loading']
       insertTpl: '${atwho-at}${id}'
+      startWithSpace: false
       callbacks:
         sorter: @DefaultOptions.sorter
         filter: @DefaultOptions.filter
@@ -149,6 +152,7 @@ GitLab.GfmAutoComplete =
           @Loading.template
       insertTpl: '${atwho-at}"${title}"'
       data: ['loading']
+      startWithSpace: false
       callbacks:
         beforeSave: (milestones) ->
           $.map milestones, (m) ->
@@ -169,6 +173,7 @@ GitLab.GfmAutoComplete =
           @Loading.template
       data: ['loading']
       insertTpl: '${atwho-at}${id}'
+      startWithSpace: false
       callbacks:
         sorter: @DefaultOptions.sorter
         filter: @DefaultOptions.filter
@@ -187,6 +192,7 @@ GitLab.GfmAutoComplete =
       searchKey: 'search'
       displayTpl: @Labels.template
       insertTpl: '${atwho-at}${title}'
+      startWithSpace: false
       callbacks:
         beforeSave: (merges) ->
           sanitizeLabelTitle = (title)->

spec/features/issues/gfm_autocomplete_spec.rb

+require 'rails_helper'
+
+feature 'GFM autocomplete', feature: true, js: true do
+  let(:user)    { create(:user) }
+  let(:project) { create(:project) }
+  let(:issue)   { create(:issue, project: project) }
+
+  before do
+    project.team << [user, :master]
+    login_as(user)
+    visit namespace_project_issue_path(project.namespace, project, issue)
+  end
+
+  it 'opens autocomplete menu when doesnt starts with space' do
+    sleep 2
+
+    page.within '.timeline-content-form' do
+      find('#note_note').native.send_keys('testing')
+      find('#note_note').native.send_keys('@')
+    end
+
+    expect(page).to have_selector('.atwho-view')
+  end
+end