Don't let 'ScrollEffect.reset()' set 'is_manual' to True

[gottadiveintopython]

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Fixes #7721.
This is a substitute for #7727.

The cause of the issue

KineticEffect.is_manual should be True only when there is a touch performing a scroll. But under a certain circumstance, it can be True even though no touch events are in progress. You can confirm it by watching the is_manual property while running the #7721 example and resizing the window like shown in the #7721 video.

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

KV = """
<BugScroll>:
    ScrollView:
        id: sv
        effect_cls:'ScrollEffect'
        BoxLayout:
            size_hint_y:None
            height:self.minimum_height
            orientation:'vertical'
            id:box
            always_overscroll: False
"""
Builder.load_string(KV)

class BugScroll(BoxLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        for i in range(100):
            self.ids.box.add_widget(Button(size_hint_y=None, height=55, text=str(i)))

class Program(App):
    def build(self):
        return BugScroll()
    def on_start(self):
        self.root.ids.sv.effect_y.bind(is_manual=lambda __, value: print(f"is_manual: {value}"))
        

if __name__ == '__main__':
    Program().run()

issue7721.mp4

Because of that, ScrollView uses its previous size while re-positioning its content when it gets resized. The following code is where it happens.

kivy/kivy/uix/scrollview.py

Lines 624 to 627 in 315b742

	if self.effect_x.is_manual:
	sw = vp.width - self._effect_x_start_width
	else:
	sw = vp.width - self.width

kivy/kivy/uix/scrollview.py

Lines 639 to 642 in 315b742

	if self.effect_y.is_manual:
	sh = vp.height - self._effect_y_start_height
	else:
	sh = vp.height - self.height

How is_manual ends up being True after a touch performing a scroll ends

1. When a touch performing a scroll ends, is_manual is set to False
2. The momentum of the scroll still exists so ScrollEffect.value will be updated for a while.
3. If ScrollEffect.value exceeds the bounds(ScrollEffect.min,ScrollEffect.max), ScrollEffect.reset() gets called and is_manual will be set to True.

Why the issue doesn't occur when DampedScrollEffect is being used

It overwrites on_value() but doesn't call super().on_value(), which is the one who calls reset().

kivy/kivy/effects/scroll.py

Lines 96 to 109 in fe9cd88

	def on_value(self, *args):
	scroll_min = self.min
	scroll_max = self.max
	if scroll_min > scroll_max:
	scroll_min, scroll_max = scroll_max, scroll_min
	if self.value < scroll_min:
	self.overscroll = self.value - scroll_min
	self.reset(scroll_min)
	elif self.value > scroll_max:
	self.overscroll = self.value - scroll_max
	self.reset(scroll_max)
	else:
	self.scroll = self.value

[gottadiveintopython]

@HeaTTheatR
This might affect RouletteScrollEffect. StiffScrollEffect probably doesn't get affected for the same reason DampedScrollEffect doesn't.

[Zen-CODE]

@gottadiveintopython Nice sleuthing. Works for me :-)