`winit::TabLeft`/`Event::Suspended` doesn't function as intended on WASM about bevy

[daxpedda]

I was investigating if there is an issue in Winit or if Winit can do something here, just dropping my results here.

So my assumption is that the reason why Event::Suspended is only received when going back to the tab is because in general all Winit events in Bevy are cached in handled in WindowEvent::RedrawRequested. But this event has to be handled in the event loop directly and not when WindowEvent::RedrawRequested is reached (which would be in the next browser event loop tick).

Unfortunately, the spec doesn't specify anymore in which case a browser is allowed or not allowed to freeze a page (and discard it afterwards). To prevent browsers based on Chromium (the only engine currently implementing the Page Lifecycle API) from freezing the page, see this document by Chromium outlining their heuristic.

In the future the spec might officially introduce an opt-out method with the Screen Wake Lock API, see WICG/page-lifecycle#31.

[Nul-led]

Im not really sure if this should be part of bevy to be honest. The solution mentioned uses the fact that web workers aren't throttled by major browsers (unlike timers in content scripts), however this is (as far as im aware) not standardized and up to the browser for implementation. So it is essentially a temporary fix not a solution as browsers could very well change this behavior at any point. Which leads me to believe that it is not a good idea to rely on this in bevy.

However it might be a good idea to run app.update() whenever the browser fires an visibilitychange event.

Additionally my solution relies on unsafe casting and will only work in a single threaded environment so it isn't entirely future proof in that aspect either (see multithreading proposal for WebAssembly).

Tldr;
I believe this should be implemented in an independent crate or at least with an optional feature.

[Nul-led]

Published https://crates.io/crates/bevy_web_keepalive if someone needs a solution until this is fixed upstream (if this gets fixed at all)

[Nul-led]

tho @happydpc this might not work for you as this only runs the Main schedule, so the renderer subapp wouldn't be affected.

[simbleau]

Yeah my issues are fixed with that plugin

[simbleau]

In the meantime, for anyone visiting this issue, just use https://crates.io/crates/bevy_web_keepalive

It solves the issue for all browsers by deploying a Web worker which runs the Main schedule once every (configurable) interval when the window is suspended.