Example of a RecycleView that keeps the current view position when new data is added, unless we are at the very bottom, in which case we follow the scroll.

fixed_rv.py

from random import sample, randint
from string import ascii_lowercase
from time import asctime

from kivy.app import App
from kivy.factory import Factory
from kivy.lang import Builder
from kivy.properties import NumericProperty, ListProperty
from kivy.clock import Clock


KV = """
BoxLayout:
    orientation: 'vertical'
    Label:
        size_hint_y: None
        height: self.texture_size[1]
        text:
            F'''height: {rv.height}
            scrollable_distance: {rv.scrollable_distance}
            distance_to_top: {rv.distance_to_top}
            scroll_y: {rv.scroll_y}
            len items: {len(rv.data)}
            '''

    ToggleButton:
        id: active
        state: 'down'
        text: 'active'
        size_hint_y: None
        height: '50dp'

    ToggleButton:
        id: show
        state: 'down'
        text: 'active'
        size_hint_y: None
        height: '50dp'

    FixedRecycleView:
        id: rv
        data: app.data if show.state == 'down' else []
        viewclass: 'Label'
        scrollable_distance: box.height - self.height

        RecycleBoxLayout:
            id: box
            orientation: 'vertical'
            size_hint_y: None
            height: self.minimum_height
            default_size: 0, 48
"""


class FixedRecycleView(Factory.RecycleView):
    distance_to_top = NumericProperty()
    scrollable_distance = NumericProperty()

    def on_scrollable_distance(self, *args):
        if self.scroll_y > 0:
            self.scroll_y = (self.scrollable_distance - self.distance_to_top) / self.scrollable_distance

    def on_scroll_y(self, *args):
        self.distance_to_top = (1 - self.scroll_y) * self.scrollable_distance


class Application(App):
    pending_data = ListProperty()
    data = ListProperty()

    def build(self):
        Clock.schedule_interval(self.add_log, .01)
        Clock.schedule_interval(self.flush_pending_data, .250)
        return Builder.load_string(KV)

    def flush_pending_data(self, *args):
        if self.pending_data:
            pending_data, self.pending_data = self.pending_data, []
            self.data.extend(pending_data)
    
    def add_log(self, dt):
        if self.root.ids.active.state == 'down':
            self.pending_data.append({
                'text': f"[{asctime()}]: {' '.join(''.join(sample(ascii_lowercase, randint(5, 20))) for i in range(10))}"
            })


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