ScrollView: Match scroll effect stop condition to start condition.

[matham]

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Fixes #7207.

Currently the stop condition is such that the effect stop may be executed even if the effect start was not called. Because the start and stop were guarded under different conditions. This makes them equal.

[tshirtman]

Doesn't seem to fix #7207 for me, if i drag the bar first, without touching anything before that, it crashes 100% of the time.

[tshirtman]

Probably not the prettiest solution, but

--- a/kivy/effects/kinetic.py
+++ b/kivy/effects/kinetic.py
@@ -145,7 +145,7 @@ def update(self, val, t=None):
         See :meth:`start` for the arguments.
         '''
         t = t or time()
-        distance = val - self.history[-1][1]
+        distance = val - (self.history[-1][1] if self.history else 0)
         self.apply_distance(distance)
         self.history.append((t, val))
         if len(self.history) > self.max_history:
diff --git a/kivy/effects/scroll.py b/kivy/effects/scroll.py
index 7a722b8d7..f35b5c39b 100644
--- a/kivy/effects/scroll.py
+++ b/kivy/effects/scroll.py
@@ -113,7 +113,7 @@ def start(self, val, t=None):
         return super(ScrollEffect, self).start(val, t)
 
     def update(self, val, t=None):
-        self.displacement += abs(val - self.history[-1][1])
+        self.displacement += abs(val - (self.history[-1][1] if self.history else 0))
         return super(ScrollEffect, self).update(val, t)
 
     def stop(self, val, t=None):

seems to solve it for me without side effects.

[matham]

I overlooked that crash because when I translated from kivymd I didn't fully match the kivymd rules. But it should now be fixed for scroll move as well.

[matham]

The history fix would definitely fix it, but it doesn't fix the underlying issue.

[tshirtman]

Right, but unless we have time to properly fix (and i can't promise i will) i would rather have that.

[matham]

Oh, yeah, my recent commit should have fixed it completely.