Fix handling of small/big fuzzyval's in NeighborList.

This fixes an exception when having a really big or small zoom (e.g. 0) and
then using +/-.

Fixes #361.

qutebrowser/test/utils/usertypes/test_neighborlist.py

@@ -355,6 +355,30 @@ class SnapInTests(unittest.TestCase):
         self.assertEqual(self.nl.previtem(), 1)
         self.assertEqual(self.nl._idx, 2)
 
+    def test_too_big_next(self):
+        """Test fuzzyval/next with a value bigger than any in the list."""
+        self.nl.fuzzyval = 30
+        self.assertEqual(self.nl.nextitem(), 20)
+        self.assertEqual(self.nl._idx, 0)
+
+    def test_too_big_prev(self):
+        """Test fuzzyval/prev with a value bigger than any in the list."""
+        self.nl.fuzzyval = 30
+        self.assertEqual(self.nl.previtem(), 20)
+        self.assertEqual(self.nl._idx, 0)
+
+    def test_too_small_next(self):
+        """Test fuzzyval/next with a value smaller than any in the list."""
+        self.nl.fuzzyval = 0
+        self.assertEqual(self.nl.nextitem(), 1)
+        self.assertEqual(self.nl._idx, 2)
+
+    def test_too_small_prev(self):
+        """Test fuzzyval/prev with a value smaller than any in the list."""
+        self.nl.fuzzyval = 0
+        self.assertEqual(self.nl.previtem(), 1)
+        self.assertEqual(self.nl._idx, 2)
+
 
 if __name__ == '__main__':
     unittest.main()

qutebrowser/utils/usertypes.py

@@ -116,8 +116,14 @@ class NeighborList(collections.abc.Sequence):
         op = operator.le if offset < 0 else operator.ge
         items = [(idx, e) for (idx, e) in enumerate(self._items)
                  if op(e, self.fuzzyval)]
-        close_item = min(items, key=lambda tpl: abs(self.fuzzyval - tpl[1]))
-        self._idx = close_item[0]
+        if items:
+            item = min(items, key=lambda tpl: abs(self.fuzzyval - tpl[1]))
+        else:
+            sorted_items = sorted([(idx, e) for (idx, e) in
+                                   enumerate(self.items)])
+            idx = 0 if offset < 0 else -1
+            item = sorted_items[idx]
+        self._idx = item[0]
         return self.fuzzyval not in self._items
 
     def _get_new_item(self, offset):