Add spec to show sticky button on mobile screens

Also remove some unnecessary css lines that were causing some unrelated specs to fail.

app/assets/stylesheets/layout.scss

@@ -446,8 +446,6 @@ header {
   background: $brand;
   border-bottom: 1px solid $border;
   margin-bottom: $line-height;
-  position: relative;
-  z-index: 10;
 
   .selected {
     border-bottom: 1px solid #fff;

spec/features/proposals_spec.rb

@@ -178,6 +178,27 @@ feature "Proposals" do
     end
   end
 
+  context "Show on mobile screens" do
+
+    before do
+      Capybara.page.driver.browser.manage.window.resize_to(640, 480)
+    end
+
+    after do
+      Capybara.page.driver.browser.manage.window.maximize
+    end
+
+    scenario "Show support button sticky at bottom", :js do
+      proposal = create(:proposal)
+      visit proposal_path(proposal)
+
+      within("#proposal_sticky") do
+        expect(page).to have_css(".is-stuck")
+        expect(page).not_to have_css(".is-anchored")
+      end
+    end
+  end
+
   context "Embedded video" do
 
     scenario "Show YouTube video" do