diff --git a/druid-shell/src/backend/mac/window.rs b/druid-shell/src/backend/mac/window.rs
index 836f49543f..a212412cf3 100644
--- a/druid-shell/src/backend/mac/window.rs
+++ b/druid-shell/src/backend/mac/window.rs
@@ -1424,6 +1424,7 @@ unsafe impl HasRawWindowHandle for WindowHandle {
 }
 
 unsafe impl Send for IdleHandle {}
+unsafe impl Sync for IdleHandle {}
 
 impl IdleHandle {
     fn add_idle(&self, idle: IdleKind) {
diff --git a/druid-shell/src/backend/web/window.rs b/druid-shell/src/backend/web/window.rs
index 92a0326f52..4dfd1ddfc1 100644
--- a/druid-shell/src/backend/web/window.rs
+++ b/druid-shell/src/backend/web/window.rs
@@ -688,6 +688,7 @@ impl WindowHandle {
 }
 
 unsafe impl Send for IdleHandle {}
+unsafe impl Sync for IdleHandle {}
 
 impl IdleHandle {
     /// Add an idle handler, which is called (once) when the main thread is idle.
diff --git a/druid-shell/src/backend/windows/window.rs b/druid-shell/src/backend/windows/window.rs
index 1a0c64bde7..e580325ce6 100644
--- a/druid-shell/src/backend/windows/window.rs
+++ b/druid-shell/src/backend/windows/window.rs
@@ -2236,6 +2236,7 @@ impl WindowHandle {
 
 // There is a tiny risk of things going wrong when hwnd is sent across threads.
 unsafe impl Send for IdleHandle {}
+unsafe impl Sync for IdleHandle {}
 
 impl IdleHandle {
     /// Add an idle handler, which is called (once) when the message loop