anchor visual scroll position when adding widgets above the viewport

[fuzzybritches0]

I need to load Markdown widgets above the viewport and want to implement on demand loading of Markdown widgets, since loading many Markdown widgets onto one container can make the app very slow and cumbersome to use.
I couldn't find a way to have the visual scroll position persist when adding widgets above the viewport. So I tried to override some base methods but I couldn't make it workI Then I looked closer at the code and found a way. I really don't know if this is the right way to do it, or if there is a better way already. So I'm reaching out for help. But if it is, and if useful to others, I'll be happy to have it pulled.

https://github.com/fuzzybritches0/textual/tree/anchor_visual

Cheers

[willmcgugan]

I'm afraid its not clear to me what you are looking for. I'm assuming your fork is AI generated, but it isn't going to help me understand your issue.

I'd suggest an MRE (a short self contained code example) or possibly a diagram, and and a more detailed explanation...

[fuzzybritches0]

Okay. Let me try to explain it in different words and then show you how I use anchor_visual() in a practical example:

If I have a set of Markdown widgets mounted on a VerticalScroll container and I mount another Markdown widget on top of the list - VerticalScroll.mount(Markdown, before=0), VerticalScroll.scroll_y remains the same, and this leads to the effect that there is now a different Markdown container at VerticalScroll.scroll_y, which I don't want. I want to have the exact same visual position remain in the viewport, which requires me to change scroll_y. Listening to Resize on VerticalScroll, calculating the difference for scroll_y, and then adjusting scroll_y, was my first attempt at trying to solve this issue. However, the Resize message gets send from Compositor._arrange_root.add_widget in _compositor.py, which does let me calculate the new scroll_y I want, but I won't be able to adjust it in time to avoid the 'jerking' between scroll_y and the newly calculated offset scroll_y.

Here is my practical example:

I want to solve the following issue:

1. I have a chat message view with lots of messages that need to get loaded at app start. This can take a very long time. If I load them reversed(), the user sees the last message first, almost instantly. Then I load the other messages. If anchor() is set and the user does not scroll up, all is good. However, if the user scrolls up, while the messages are loading, the user labours under the impression that now the VerticalScroll is magically scrolling up.

from textual import work
from textual.containers import VerticalScroll
from textual.widgets import Markdown
from chat import load_messages

class ChatView(VerticalScroll):
    def __init__(self, chat_id: str) -> None:
        super().__init__()
        self.messages = load_messages(chat_id)

    #We set the anchor`
    self.anchor()

    #We also set the visual anchor
    self.anchor_visual()

    def load(self) -> None:
        self.load_messages()

    @work
    async def load_messages(self) -> None:
        # first we mount the last message and focus it
        await self.mount(Markdown, self.messages[-1])
        self.children[0].focus(scroll_visible=False)

        #Then we mount the rest
        for message in reversed(self.messages[:-1]):
            await self.mount(Markdown, message), before=0)

You assume wrong! The fork was not AI generated. I enjoy writing code. :-)
I hope this clears all up.

Cheers

[fuzzybritches0]

Sorry, I noticed that my initial testing must have been flawed. Upon trying to implement on-demand loading and unloading of widgets again in my app, I noticed that my initial assumptions are wrong and that my implementation is not doing what I claim in the documentation. So this goes without saying, of course, that this is not pull request worthy. Thanks anyways!

Cheers