Clicks lost in reactive mode

[Azorlogh]

Button clicks are sometimes lost in reactive mode.
To reproduce, run this example:

use bevy::{prelude::*, winit::WinitSettings};
use bevy_egui::{EguiContexts, EguiPlugin};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(EguiPlugin)
        .insert_resource(WinitSettings::desktop_app())
        .add_systems(Update, ui_example_system)
        .run();
}

fn ui_example_system(mut contexts: EguiContexts, mut count: Local<u32>) {
    egui::Window::new("Hello").show(contexts.ctx_mut(), |ui| {
        ui.label(format!("{}", *count));
        if ui.button("world").clicked() {
            *count += 1;
        }
    });
}

Click the button multiple times, and sometimes the counter will not increment

[vladbat00]

I think I've noticed it sometimes as well. I haven't got a chance to look into that myself yet, but if anyone wants to submit a PR with a fix, I'm more than happy to merge it.

[vladbat00]

Oh, btw, could you try reproducing it with the 0.29 version? There were some changes merged that fix redraws, but I'm not sure if they cover this issue.

[Azorlogh]

Just tested with 0.29, it still happens

[Azorlogh]

I found a clue:
egui ignores clicks that have been pressed for more than 0.8s. If I disable this by setting the delay 1000.0, the bug no longer appears.
This would suggest the problem is be timing related.
Without this change, even if I click super fast, the button still misses sometimes, so maybe we are somehow exceeding this click duration in a single frame or something 🤔

[Azorlogh]

Well, I think the issue is essentially caused by bevyengine/bevy#14682

The time we report to egui is time.elapsed_seconds_f64(), which is actually delayed by a few frames.
This means this sequence occurs (just an handwritten example):

-> tick           actual_time = 1.0        time.elapsed_seconds() = 0.0
-> mouse press    actual_time = 3.0        time.elapsed_seconds() = 1.0
-> mouse release  actual_time = 3.1        time.elapsed_seconds() = 3.0

Even though there was only 0.1s between press and release, the reported elapsed_seconds will be delayed by a few frames, so egui will perceive it as 2s so it will not register the click.

I can see two workarounds, though I'm not sure either of them really address the root issue:

• Have bevy_egui internally do its own time tracking, to bypass bevy's delayed timekeeping
• Make the max click duration configurable, so that we can disable it