How to accurately scroll to a specific line in ScrollArea when using show_rows()?

[unknownuser2503]

Problem

I've been trying to implement search functionality in egui by allowing the user to scroll to a specific line in a ScrollArea, using show_rows() since I am sometimes displaying a lot of lines.
I can't seem to find any way of accurately scrolling to a target line when using show_rows().

Things I've tried

The only approach I've gotten to at least scroll in a sensible way is using vertical_scroll_offset():

const DEFAULT_FONT_ID: FontId = FontId::new(14.0, FontFamily::Monospace);
...
let row_height = DEFAULT_FONT_ID.size*0.95;
let num_rows = packets.len(); // packets is a Vec<String>
let width = ui.available_size().x;

let scroll_offset = (line_of_search_occurrence as f32) * row_height;

egui::ScrollArea::vertical()
  .auto_shrink([false; 2])
  .animated(false)
  .stick_to_bottom(true)
  .enable_scrolling(true)
  .max_width(width)
  .vertical_scroll_offset(scroll_offset)
  .show_rows(ui, row_height, num_rows, |ui, row_range| {
  let content: String = row_range
    .into_iter()
    .flat_map(|i| Some(self.format_packet(packets[i]))) // self.format_packet returns a String.
    .collect::<String>();

  egui::TextEdit::multiline(&mut content.as_str())
    .font(DEFAULT_FONT_ID)
    .lock_focus(true)
    .desired_width(width)
}

With this approach however, the scroll is not accurate. It is often close if there aren't a lot of lines being shown, but when there are a lot of lines it is no where near accurate enough.

My first instinct was the row height being incorrect, so first I tried setting the scroll_offset to the following:

let scroll_offset = (line_of_search_occurrence as f32) * DEFAULT_FONT_ID.size;
let scroll_offset = (line_of_search_occurrence as f32) * ui.text_style_height(&egui::TextStyle::Body);

That didn't work, so I tested it on some repeating text to see how far off the distance scrolled was. Then I tried doing

let scroll_offset = DEFAULT_FONT_ID.size*multiplier;

and adjusted the multiplier until the distance scrolled was perfect, and I got multiplier=1.662. This however, doesn't work when just testing it on some random text with a bunch of lines. It's often close if there aren't a lot of lines, but when testing on a thousands of lines, it's no where close enough.

I've thought it might have something to do with lines wrapping and thus taking the space of 2 lines. Testing this on a big screen without any lines wrapping, that doesn't seem to solve the problem, which to me suggests that isn't a problem?

I've also tried to use some of the scroll_to* functions. Using those however, the scroll to line doesn't work at all, but I am assuming that even if I got that to work, I would run into the same problem?

Why is this approach not working, and is there a solution to the problem?

[unknownuser2503]

Found a solution!

Looking into the show_rows() function, I figured out that the actual "height" for each line is:

let row_height_with_spacing = row_height + ui.spacing().item_spacing.y;

Then you can calculate the scroll offset for the line, target_line, as so:

let scroll_offset = (target_line as f32) * row_height_with_spacing - ui.spacing().item_spacing.y;

I want the target line to be displayed close to the top, so I find the total window height and subtract 20% of that to the scroll offset:

let window_rect = ctx.input(|i: &egui::InputState| i.screen_rect());
let window_height: f32 = window_rect.max[1] - window_rect.min[1];

my_scroll_area = my_scroll_area.vertical_scroll_offset(scroll_offset - window_height * 0.2);