pushbutton.py: Fix iss 115.

Author: peterhinch

v3/docs/DRIVERS.md

@@ -423,11 +423,14 @@ Methods 1 - 4 may be called at any time. If `False` is passed for a callable,
 any existing callback will be disabled. If `None` is passed, a bound `Event` is
 created. See below for `Event` names.
 
-Class attributes:
+Class variables:
  1. `debounce_ms` Debounce time in ms. Default 50.
  2. `long_press_ms` Threshold time in ms for a long press. Default 1000.
  3. `double_click_ms` Threshold time in ms for a double-click. Default 400.
 
+ If these variables are changed, it should be done prior to instantiating the
+ class.
+
 A simple Pyboard demo:
 ```python
 from pyb import LED

v3/primitives/pushbutton.py

@@ -78,7 +78,7 @@ def _check(self, state):
 
     def _ddto(self):  # Doubleclick timeout: no doubleclick occurred
         self._dblpend = False
-        if self._supp and not self._state:
+        if self._ff and self._supp and not self._state:
             if not self._ld or (self._ld and not self._ld()):
                 launch(self._ff, self._fa)