ComboBox menu doesn't report its actual size to its closure

[abey79]

Context

Consider the following code:

#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
#![allow(rustdoc::missing_crate_level_docs)] // it's an example

use eframe::egui;

fn main() -> Result<(), eframe::Error> {
    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
    let options = eframe::NativeOptions {
        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
        ..Default::default()
    };
    eframe::run_native("My egui App", options, Box::new(|cc| Box::new(MyApp)))
}

struct MyApp;

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, ui_fn);
    }
}

fn ui_fn(ui: &mut egui::Ui) {
    egui::ComboBox::from_id_source("combo")
        .selected_text("ComboBox")
        .width(100.0)
        .show_ui(ui, |ui| {
            ui.label("Hello");
            ui.label("World");
        });
}

The combobox correctly sets the width of its menu according to width(100.0).

Let's add some longer content:

fn ui_fn(ui: &mut egui::Ui) {
    egui::ComboBox::from_id_source("combo")
        .selected_text("ComboBox")
        .width(100.0)
        .show_ui(ui, |ui| {
            ui.label("Hello");
            ui.label("World");
            ui.label("Hellooooooooooooooooooooooooo");
        });
}

The menu automatically increases its size to accommodate the longer content. Nice.

Expected

The ui passed to the closure in show_ui should report its actual size, which is presumably computed from last frame content. This is useful e.g. to know the full span of the menu, should the content want to have full-span highlighting (aka rerun's ListItem).

Actual

The ui reflects the base, unexpended width.

fn ui_fn(ui: &mut egui::Ui) {
    egui::ComboBox::from_id_source("combo")
        .selected_text("ComboBox")
        .width(100.0)
        .show_ui(ui, |ui| {
            ui.ctx()
                .debug_painter()
                .debug_rect(ui.max_rect(), egui::Color32::RED, "");

            ui.label("Hello");
            ui.label("World");
            ui.label("Hellooooooooooooooooooooooooo");
        });
}

Workaround

Manually set the width to the desired value, which requires knowing a upper bound for the content width.

fn ui_fn(ui: &mut egui::Ui) {
    egui::ComboBox::from_id_source("combo")
        .selected_text("ComboBox")
        .width(100.0)
        .show_ui(ui, |ui| {
            ui.set_width(220.0);

            ui.ctx()
                .debug_painter()
                .debug_rect(ui.max_rect(), egui::Color32::RED, "");

            ui.label("Hello");
            ui.label("World");
            ui.label("Hellooooooooooooooooooooooooo");
        });
}

[abey79]

I realise now that this might be "working as intended". If the ui would be directly set to last frame's max width, there would be no means for the menu to ever shrink if the content eventually reduces its size.

I'm not really sure how to go about this.

[emilk]

This is a very similar problem to:

• egui menus: remember size from previous frame #3974

[Myprivateclonelibrary]

Hi. When will this bug be fixed?
Maybe use Textedit