How to Scroll to the Bottom when new content arrives

[masenf]

@luka on discord asks:

Is it possible to enforce scrollToBottom on rx.scroll_area on any text content update?

[masenf]

Using the reflex-intersection-observer component, along with the scrollIntoView() function supported by most browsers, you can have a hidden 1px div at the bottom of your message list, and when it goes out of view, trigger an event handler to automatically scroll the hidden div back into view, once.

This has the nice side effect of allowing your users to scroll up in the messages without automatically scrolling back down when a new message arrives, but re-enabling the auto scroll to bottom if they do scroll it back down to the latest message.

It has one unfortunate side effect of a tiny little jitter when initially scrolling up, because the component will try to scroll back down the first time the hidden div goes out of view, even when this is initiated by user action.

pip install 'reflex-intersection-observer>=0.0.2'

import asyncio

import reflex as rx

from reflex_intersection_observer import intersection_observer


BOTTOM_ELEMENT_ID = "bottom"


class ScrollHandlingState(rx.State):
    def scroll_to_bottom(self):
        return rx.call_script(f"document.getElementById('{BOTTOM_ELEMENT_ID}').scrollIntoView()")

    def handle_non_intersect(self, _entry):
        return self.scroll_to_bottom()


class MessageGenerator(rx.State):
    messages: list[str]
    should_load: bool = False

    def on_load(self):
        self.messages = ["Hello, world!"]
        self.should_load = True
        return MessageGenerator.add_messages

    @rx.background
    async def add_messages(self):
        while self.should_load:
            async with self:
                self.messages.append(f"New message {len(self.messages) + 1}!")
                if not self.should_load:
                    break
            await asyncio.sleep(3)


def message_control_button():
    return rx.cond(
        MessageGenerator.should_load,
        rx.button("Stop", on_click=MessageGenerator.set_should_load(False)),
        rx.button("Start", on_click=[MessageGenerator.set_should_load(True), MessageGenerator.add_messages]),
    )


def index() -> rx.Component:
    return rx.vstack(
        rx.heading("Messages"),
        rx.hstack(
            message_control_button(),
            rx.icon_button(
                rx.icon("arrow_down_to_line"),
                on_click=ScrollHandlingState.scroll_to_bottom,
            ),
        ),
        rx.scroll_area(
            rx.vstack(
                rx.foreach(MessageGenerator.messages, rx.text),
                intersection_observer(
                    height="1px",
                    id=BOTTOM_ELEMENT_ID,
                    root="#scroller",
                    on_non_intersect=ScrollHandlingState.handle_non_intersect,
                    visibility="hidden",
                ),
            ),
            id="scroller",
            width="90vw",
            height="100px",
        ),
        rx.text("This is outside scroll area"),
        align="center",
    )


app = rx.App()
app.add_page(index, on_load=MessageGenerator.on_load)

Screen.Recording.2024-03-13.at.12.58.34.mov

The jittering could be fixed with another state var that tracks whether a new message has arrived and only calling scroll_to_bottom if so, then reset the new message var inside handle_non_intersect.