fix regression when going below the log zone

Since a431b68c28, when going below the max
of the log files, the position button would remain in fixed position.
This fixes it.

assets/scripts/app/tailing.coffee

@@ -51,8 +51,13 @@ class @Travis.Tailing
     return if @tail().length is 0
     offset = @window.scrollTop() - @log().offset().top
     max = @log().height() - @tail().height() + 5
-    offset = max if offset > max
-    if offset > 0
+
+    if offset > 0 && offset <= max
+      @tail().removeClass('bottom')
       @tail().addClass('scrolling')
     else
+      if offset > max
+        @tail().addClass('bottom')
+      else
+        @tail().removeClass('bottom')
       @tail().removeClass('scrolling')

assets/scripts/spec/unit/tailing_spec.coffee

@@ -71,8 +71,19 @@ test 'positionButton adds the scrolling class', ->
 
   tail.positionButton()
   equal(element.hasClass('scrolling'), true)
+  equal(element.hasClass('bottom'), false)
 
 test 'positionButton removes the scrolling class', ->
   log.offset = -> {top: 1}
   tail.positionButton()
   equal(element.hasClass('scrolling'), false)
+  equal(element.hasClass('bottom'), false)
+
+test 'positionButton sets the button as bottom', ->
+  log.offset  = -> {top: -100}
+  log.height  = -> 50
+  tail.height = -> 1
+
+  tail.positionButton()
+  equal(element.hasClass('scrolling'), false)
+  equal(element.hasClass('bottom'), true)

assets/styles/main/log.sass

@@ -121,6 +121,10 @@ pre#log
       position: fixed
       right: 32px
 
+    &.bottom
+      bottom: 45px
+      top: inherit
+
     .status
       display: inline-block
       margin-right: 1px