Fix the gap calculation algorithm to be more efficient.

We no only calculate the single line gap in the case of SingleLine
style useage. The previous code had some errors in the case statement.
This version makes more sense and seems more readable to me.

src/Yi/Buffer/HighLevel.hs

@@ -18,7 +18,7 @@ import Yi.Buffer.Normal
 import Yi.Buffer.Region
 import Yi.String
 import Yi.Window
-import Yi.Config.Misc (ScrollStyle)
+import Yi.Config.Misc (ScrollStyle(SingleLine))
 
 -- ---------------------------------------------------------------------
 -- Movement operations
@@ -401,12 +401,12 @@ snapScreenB style = do
             h <- askWindow actualLines
             r <- winRegionB
             p <- pointB
-            let gap = case pointScreenRelPosition p (regionStart r) (regionEnd r) of
-                        Above  -> 0
-                        Below  -> case style of
-                                    Nothing -> h `div` 2
-                                    _       -> h - 1
-                        Within -> 0 -- Impossible but handle it anyway
+            let gap = case style of
+                        Just SingleLine -> case pointScreenRelPosition p (regionStart r) (regionEnd r) of
+                                             Above  -> 0
+                                             Below  -> h - 1
+                                             Within -> 0 -- Impossible but handle it anyway
+                        _               -> h `div` 2
             i <- indexOfSolAbove gap
             f <- fromMark <$> askMarks
             setMarkPointB f i