Crash on macOS when using Window::set_ime_position from another thread

[amatho]

Calling Window::set_ime_position in another thread crashes the program on macOS. The issue is not present on Linux (and presumably not on Windows either). I'm running on aarch64 but I would guess this also happens on x86_64. Minimal repro:

use std::{sync::Arc, thread, time::Duration};

use winit::{
    dpi::LogicalPosition,
    event::{Event, WindowEvent},
    event_loop::EventLoop,
    window::WindowBuilder,
};

fn main() {
    let event_loop = EventLoop::new();
    let window = WindowBuilder::new().build(&event_loop).unwrap();
    let window = Arc::new(window);

    thread::spawn(move || {
        // Wait for event loop to initialize
        thread::sleep(Duration::from_secs(1));

        // This crashes the program on macOS
        window.set_ime_position(LogicalPosition::new(42, 42));
    });

    event_loop.run(move |event, _, control_flow| {
        control_flow.set_wait();

        if let Event::WindowEvent {
            event: WindowEvent::CloseRequested,
            ..
        } = event
        {
            control_flow.set_exit();
        }
    });
}

Backtrace from LLDB:

* thread #4, stop reason = EXC_BREAKPOINT (code=1, subcode=0x19e4ac924)
  * frame #0: 0x000000019e4ac924 libdispatch.dylib`_dispatch_assert_queue_fail + 120
    frame #1: 0x000000019e4ac8ac libdispatch.dylib`dispatch_assert_queue + 196
    frame #2: 0x00000001a7df26a4 HIToolbox`islGetInputSourceListWithAdditions + 160
    frame #3: 0x00000001a7df4ccc HIToolbox`isValidateInputSourceRef + 92
    frame #4: 0x00000001a7df4b8c HIToolbox`TSMGetInputSourceProperty + 44
    frame #5: 0x00000001a7e0b7b8 HIToolbox`utInputSourceIsKeyboardInputMethod + 36
    frame #6: 0x00000001a7e1059c HIToolbox`utInvalidateThisDocsInputMethodClientGeometry + 48
    frame #7: 0x00000001a7e10510 HIToolbox`TSMInvalidateClientGeometry + 208
    frame #8: 0x00000001a1b27274 AppKit`-[NSTextInputContext invalidateCharacterCoordinates] + 60
    frame #9: 0x0000000100086d84 winit-macos-ime-crash`_$LT$$LP$$RP$$u20$as$u20$objc2..message..MessageArguments$GT$::__invoke::h435e508f485b56a3(imp=(libobjc.A.dylib`objc_msgSend), obj=0x0000600002c03900, sel=Sel @ 0x000000017011e9c0, (null)=<unavailable>) at mod.rs:390:26
    frame #10: 0x0000000100087a78 winit-macos-ime-crash`objc2::message::platform::send_unverified::_$u7b$$u7b$closure$u7d$$u7d$::h14e3c02a894abdde at mod.rs:36:33
    frame #11: 0x0000000100086b0c winit-macos-ime-crash`objc2::message::conditional_try::hd7f23106e54cd850(f=<unavailable>) at mod.rs:30:5
    frame #12: 0x0000000100087258 winit-macos-ime-crash`objc2::message::platform::send_unverified::h15e9a754cf8be473(receiver=0x0000600002c03900, sel=Sel @ 0x000000017011ea40, args=<unavailable>) at mod.rs:36:14
    frame #13: 0x0000000100038164 winit-macos-ime-crash`objc2::message::MessageReceiver::send_message::h90f62d9a95d2ee1a(self=0x0000600002c03900, sel=Sel @ 0x000000017011eaa8, args=<unavailable>) at mod.rs:195:46
    frame #14: 0x000000010003c1fc winit-macos-ime-crash`winit::platform_impl::platform::appkit::text_input_context::NSTextInputContext::invalidateCharacterCoordinates::h781262856ac9771d(self=0x0000600002c03900) at __rewrite_self_arg.rs:42:21
    frame #15: 0x000000010001ed44 winit-macos-ime-crash`winit::platform_impl::platform::view::WinitView::set_ime_position::h0c0e80aaf57481f7(self=0x000000010040f100, position=(x = 42, y = 42)) at view.rs:940:9
    frame #16: 0x000000010000b70c winit-macos-ime-crash`winit::platform_impl::platform::window::WinitWindow::set_ime_position::h970e43686f4e6761(self=0x000000010053ab90, spot=Position @ 0x000000017011ebf0) at window.rs:1169:9
    frame #17: 0x0000000100005a98 winit-macos-ime-crash`winit::window::Window::set_ime_position::h77c1fa87ea2725a2(self=0x00006000002306f0, position=(x = 42, y = 42)) at window.rs:1027:9
    frame #18: 0x0000000100008f68 winit-macos-ime-crash`winit_macos_ime_crash::main::_$u7b$$u7b$closure$u7d$$u7d$::h9e9d3fab29932e26 at main.rs:20:9
...

Would have loved to create a PR, but I unfortunately have no experience with Objective-C or OS windowing APIs in general. Hope this helps!

[kchibisov]

Seems like we need to wrap the call to execute it on the main thread (we already do that for multiple stuff on macOS).

[amatho]

I've tried my hand at fixing this in #2757, feel free to give feedback! :)