tauri-apps
iOS Scroll freezes entire Rust event loop
iOS Scroll freezes entire Rust event loop
Problem
On iOS, all EventLoopProxy events are blocked while the user is scrolling. Events queue up and deliver only after scrolling stops.
This breaks every Tao-based iOS app that needs real-time updates during scroll.
Affected (anything using EventLoopProxy to reach the main thread):
- Signal/state updates
- Async task completions
- Cross-thread UI communication
The Rust code runs, but the main thread never receives events until scroll stops.
Screen Recording of Issue
https://github.com/rapporian/ios-scroll-blocks-rust/raw/refs/heads/main/ScreenRecording.mp4
(Red dot = Rust, Green dot = JavaScript. Watch the red dot freeze during scroll while green continues.)
Reproduction
Repo: https://github.com/rapporian/ios-scroll-blocks-rust
Scroll the page. Watch the Rust-driven animation freeze while JavaScript requestAnimationFrame continues.
Root Cause
iOS run loop modes:
| Mode | When Active | DefaultMode Observers |
CommonModes Observers |
|---|---|---|---|
kCFRunLoopDefaultMode |
Normal | ✅ Fire | ✅ Fire |
UITrackingRunLoopMode |
Scrolling | ❌ Don't fire | ✅ Fire |
The run loop observers are registered for kCFRunLoopDefaultMode only. During scroll, iOS switches to UITrackingRunLoopMode — observers aren't registered for that mode, so they don't fire and events don't get processed.
Related
https://github.com/DioxusLabs/dioxus/issues/5091
@FabianLars / @lucasfernog any chance you guys can take a look at this? It's a small fix for a pretty show-stopper issue for mobile dev (stalling futures, missing timers, etc)
can/did you test #1163 yourself? Looking at winit's code and PRs it may not be that easy and if it is we may have a hard time upstreaming this into winit.
Definately need your guys help w/ this ━ whatever the right path is, this PR (when validated) or another path.
As of right now, all interactive content freezes which is a non-starter to ship product w/ any kind of visual animations on iOS.