10,000 Textbox in Scroll is completely unresponsive to scrolling with mouse wheel/trackpad

[maan2003]

use druid::widget::{Flex, TextBox};
use druid::{AppLauncher, Widget, WidgetExt, WindowDesc};

pub fn main() {
    let window = WindowDesc::new(build_widget());
    AppLauncher::with_window(window)
        .log_to_console()
        .launch("".to_string())
        .expect("launch failed");
}

fn build_widget() -> impl Widget<String> {
    let mut col = Flex::column();
    for _ in 0..10000 {
        col.add_child(TextBox::new());
    }
    col.scroll().vertical()
}

[niklaskorz]

Encountered the same in https://github.com/niklaskorz/lyranos. If I understand it correctly (please correct me if I'm wrong), the current flow is:

1. TextBox::paint calls Scroll::paint
2. Scroll::paint calls ClipBox::paint
3. ClipBox::paint calls child.paint_raw (and at some point TextLayout::draw which draws its whole content)

The problem is that the child is not aware of its clipped viewport and thus cannot decide to limit its rendering to certain parts. Would it be possible to somehow retrieve the clip shape from the context, so the widget can determine what has to be rendered? If that were the case, TextLayout::draw could be limited to lines that are covered by the clip shape. Of course that'd mean that the TextLayout (and consequently PietTextLayoutBuilder and TextStorage::add_attributes) would have to be limitted to covered lines as well, but that again would allow a huge performance boost for syntax highlighting (which is my use case right now).