Don't allow event loops to run in parallel

src/platform_impl/web/event_loop/mod.rs

@@ -8,9 +8,7 @@ pub use self::window_target::EventLoopWindowTarget;
 
 use super::{backend, device, window};
 use crate::event::Event;
-use crate::event_loop::{
-    ControlFlow, EventLoopBuilder, EventLoopWindowTarget as RootEventLoopWindowTarget,
-};
+use crate::event_loop::{ControlFlow, EventLoopWindowTarget as RootEventLoopWindowTarget};
 
 use std::marker::PhantomData;
 
@@ -35,7 +33,7 @@ impl<T> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {
-        self.spawn_inner(event_handler);
+        self.spawn_inner(event_handler, false);
 
         // Throw an exception to break out of Rust execution and use unreachable to tell the
         // compiler this function won't return, giving it a return type of '!'
@@ -50,11 +48,10 @@ impl<T> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {
-        EventLoopBuilder::<T>::allow_event_loop_recreation();
-        self.spawn_inner(event_handler);
+        self.spawn_inner(event_handler, true);
     }
 
-    fn spawn_inner<F>(self, mut event_handler: F)
+    fn spawn_inner<F>(self, mut event_handler: F, event_loop_recreation: bool)
     where
         F: 'static + FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {
@@ -63,9 +60,10 @@ impl<T> EventLoop<T> {
             _marker: PhantomData,
         };
 
-        self.elw.p.run(Box::new(move |event, flow| {
-            event_handler(event, &target, flow)
-        }));
+        self.elw.p.run(
+            Box::new(move |event, flow| event_handler(event, &target, flow)),
+            event_loop_recreation,
+        );
     }
 
     pub fn create_proxy(&self) -> EventLoopProxy<T> {

src/platform_impl/web/event_loop/runner.rs

@@ -35,6 +35,7 @@ type OnEventHandle<T> = RefCell<Option<EventListenerHandle<dyn FnMut(T)>>>;
 
 pub struct Execution<T: 'static> {
     runner: RefCell<RunnerEnum<T>>,
+    event_loop_recreation: Cell<bool>,
     events: RefCell<VecDeque<EventWrapper<T>>>,
     id: RefCell<u32>,
     window: web_sys::Window,
@@ -139,6 +140,7 @@ impl<T: 'static> Shared<T> {
     pub fn new() -> Self {
         Shared(Rc::new(Execution {
             runner: RefCell::new(RunnerEnum::Pending),
+            event_loop_recreation: Cell::new(false),
             events: RefCell::new(VecDeque::new()),
             #[allow(clippy::disallowed_methods)]
             window: web_sys::window().expect("only callable from inside the `Window`"),
@@ -649,6 +651,9 @@ impl<T: 'static> Shared<T> {
         // * For each undropped `Window`:
         //     * The `register_redraw_request` closure.
         //     * The `destroy_fn` closure.
+        if self.0.event_loop_recreation.get() {
+            crate::event_loop::EventLoopBuilder::<T>::allow_event_loop_recreation();
+        }
     }
 
     // Check if the event loop is currently closed
@@ -691,6 +696,10 @@ impl<T: 'static> Shared<T> {
             DeviceEvents::Never => false,
         }
     }
+
+    pub fn event_loop_recreation(&self, allow: bool) {
+        self.0.event_loop_recreation.set(allow)
+    }
 }
 
 pub(crate) enum EventWrapper<T: 'static> {

src/platform_impl/web/event_loop/window_target.rs

@@ -69,7 +69,8 @@ impl<T> EventLoopWindowTarget<T> {
         EventLoopProxy::new(self.runner.clone())
     }
 
-    pub fn run(&self, event_handler: Box<runner::EventHandler<T>>) {
+    pub fn run(&self, event_handler: Box<runner::EventHandler<T>>, event_loop_recreation: bool) {
+        self.runner.event_loop_recreation(event_loop_recreation);
         self.runner.set_listener(event_handler);
     }