Fixed issue with dashboard/issues not filtering by milestone

Closes #15128

app/assets/javascripts/milestone_select.js.coffee

@@ -85,15 +85,21 @@ class @MilestoneSelect
           # display:block overrides the hide-collapse rule
           $value.removeAttr('style')
         clicked: (selected) ->
+          page = $('body').data 'page'
+          isIssueIndex = page is 'projects:issues:index'
+          isMRIndex = page is page is 'projects:merge_requests:index'
+
           if $dropdown.hasClass 'js-filter-bulk-update'
             return
 
-          if $dropdown.hasClass('js-filter-submit')
+          if $dropdown.hasClass('js-filter-submit') and (isIssueIndex or isMRIndex)
             if selected.name?
               selectedMilestone = selected.name
             else
               selectedMilestone = ''
             Issues.filterResults $dropdown.closest('form')
+          else if $dropdown.hasClass('js-filter-submit')
+            $dropdown.closest('form').submit()
           else
             selected = $selectbox
               .find('input[type="hidden"]')

spec/features/dashboard_issues_spec.rb

+require 'spec_helper'
+
+describe "Dashboard Issues filtering", feature: true, js: true do
+  let(:user)      { create(:user) }
+  let(:project)   { create(:project) }
+  let(:milestone) { create(:milestone, project: project) }
+
+  context 'filtering by milestone' do
+    before do
+      project.team << [user, :master]
+      login_as(user)
+
+      create(:issue, project: project, author: user, assignee: user)
+      create(:issue, project: project, author: user, assignee: user, milestone: milestone)
+
+      visit_issues
+    end
+
+    it 'should show all issues with no milestone' do
+      show_milestone_dropdown
+
+      click_link 'No Milestone'
+
+      expect(page).to have_selector('.issue', count: 1)
+    end
+
+    it 'should show all issues with any milestone' do
+      show_milestone_dropdown
+
+      click_link 'Any Milestone'
+
+      expect(page).to have_selector('.issue', count: 2)
+    end
+
+    it 'should show all issues with the selected milestone' do
+      show_milestone_dropdown
+
+      page.within '.dropdown-content' do
+        click_link milestone.title
+      end
+
+      expect(page).to have_selector('.issue', count: 1)
+    end
+  end
+
+  def show_milestone_dropdown
+    click_button 'Milestone'
+    expect(page).to have_selector('.dropdown-content', visible: true)
+  end
+
+  def visit_issues
+    visit issues_dashboard_path
+  end
+end