Make infinite scroll feature work on any size screen.

features/infinite_scroll.feature

@@ -6,7 +6,8 @@ Feature: infinite scroll
     
     Background:
       Given many posts from alice for bob
-      When I sign in as "bob@bob.bob"
+      And I resize my window to 800x600
+      And I sign in as "bob@bob.bob"
 
     Scenario: on the main stream by activity
       Then I should see 15 posts

features/step_definitions/custom_web_steps.rb

@@ -180,3 +180,9 @@ end
 Then /^the notification dropdown should be visible$/ do
   find(:css, "#notification_dropdown").should be_visible
 end
+
+When /^I resize my window to 800x600$/ do
+  page.execute_script <<-JS
+    window.resizeTo(800,600);
+  JS
+end
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