microsoft · cosmic-pixel-painter · Jan 11, 2026 · Jan 11, 2026 · Jan 11, 2026 · Jan 11, 2026
diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
@@ -15096,7 +15096,7 @@ interface GlobalEventHandlersEventMap {
     "durationchange": Event;
     "emptied": Event;
     "ended": Event;
-    "error": ErrorEvent;
+    "error": Event;
     "focus": FocusEvent;
     "focusin": FocusEvent;
     "focusout": FocusEvent;
@@ -15237,7 +15237,7 @@ interface GlobalEventHandlers {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
     onended: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-    onerror: OnErrorEventHandler;
+    onerror: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
     onfocus: ((this: GlobalEventHandlers, ev: FocusEvent) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */
@@ -39460,6 +39460,7 @@ interface WindowEventMap extends GlobalEventHandlersEventMap, WindowEventHandler
     "devicemotion": DeviceMotionEvent;
     "deviceorientation": DeviceOrientationEvent;
     "deviceorientationabsolute": DeviceOrientationEvent;
+    "error": ErrorEvent;
     "gamepadconnected": GamepadEvent;
     "gamepaddisconnected": GamepadEvent;
     "orientationchange": Event;
@@ -39612,6 +39613,8 @@ interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandler
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
      */
     ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+    onerror: OnErrorEventHandler;
     /**
      * @deprecated
      *
@@ -41794,6 +41797,8 @@ declare var ondeviceorientation: ((this: Window, ev: DeviceOrientationEvent) =>
  * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
  */
 declare var ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+declare var onerror: OnErrorEventHandler;
 /**
  * @deprecated
  *
@@ -42174,8 +42179,6 @@ declare var ondurationchange: ((this: Window, ev: Event) => any) | null;
 declare var onemptied: ((this: Window, ev: Event) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
 declare var onended: ((this: Window, ev: Event) => any) | null;
-/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-declare var onerror: OnErrorEventHandler;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
 declare var onfocus: ((this: Window, ev: FocusEvent) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */

diff --git a/baselines/ts5.5/dom.generated.d.ts b/baselines/ts5.5/dom.generated.d.ts
@@ -15082,7 +15082,7 @@ interface GlobalEventHandlersEventMap {
     "durationchange": Event;
     "emptied": Event;
     "ended": Event;
-    "error": ErrorEvent;
+    "error": Event;
     "focus": FocusEvent;
     "focusin": FocusEvent;
     "focusout": FocusEvent;
@@ -15223,7 +15223,7 @@ interface GlobalEventHandlers {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
     onended: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-    onerror: OnErrorEventHandler;
+    onerror: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
     onfocus: ((this: GlobalEventHandlers, ev: FocusEvent) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */
@@ -39434,6 +39434,7 @@ interface WindowEventMap extends GlobalEventHandlersEventMap, WindowEventHandler
     "devicemotion": DeviceMotionEvent;
     "deviceorientation": DeviceOrientationEvent;
     "deviceorientationabsolute": DeviceOrientationEvent;
+    "error": ErrorEvent;
     "gamepadconnected": GamepadEvent;
     "gamepaddisconnected": GamepadEvent;
     "orientationchange": Event;
@@ -39586,6 +39587,8 @@ interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandler
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
      */
     ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+    onerror: OnErrorEventHandler;
     /**
      * @deprecated
      *
@@ -41768,6 +41771,8 @@ declare var ondeviceorientation: ((this: Window, ev: DeviceOrientationEvent) =>
  * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
  */
 declare var ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+declare var onerror: OnErrorEventHandler;
 /**
  * @deprecated
  *
@@ -42148,8 +42153,6 @@ declare var ondurationchange: ((this: Window, ev: Event) => any) | null;
 declare var onemptied: ((this: Window, ev: Event) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
 declare var onended: ((this: Window, ev: Event) => any) | null;
-/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-declare var onerror: OnErrorEventHandler;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
 declare var onfocus: ((this: Window, ev: FocusEvent) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */

diff --git a/baselines/ts5.6/dom.generated.d.ts b/baselines/ts5.6/dom.generated.d.ts
@@ -15093,7 +15093,7 @@ interface GlobalEventHandlersEventMap {
     "durationchange": Event;
     "emptied": Event;
     "ended": Event;
-    "error": ErrorEvent;
+    "error": Event;
     "focus": FocusEvent;
     "focusin": FocusEvent;
     "focusout": FocusEvent;
@@ -15234,7 +15234,7 @@ interface GlobalEventHandlers {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
     onended: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-    onerror: OnErrorEventHandler;
+    onerror: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
     onfocus: ((this: GlobalEventHandlers, ev: FocusEvent) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */
@@ -39457,6 +39457,7 @@ interface WindowEventMap extends GlobalEventHandlersEventMap, WindowEventHandler
     "devicemotion": DeviceMotionEvent;
     "deviceorientation": DeviceOrientationEvent;
     "deviceorientationabsolute": DeviceOrientationEvent;
+    "error": ErrorEvent;
     "gamepadconnected": GamepadEvent;
     "gamepaddisconnected": GamepadEvent;
     "orientationchange": Event;
@@ -39609,6 +39610,8 @@ interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandler
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
      */
     ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+    onerror: OnErrorEventHandler;
     /**
      * @deprecated
      *
@@ -41791,6 +41794,8 @@ declare var ondeviceorientation: ((this: Window, ev: DeviceOrientationEvent) =>
  * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
  */
 declare var ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+declare var onerror: OnErrorEventHandler;
 /**
  * @deprecated
  *
@@ -42171,8 +42176,6 @@ declare var ondurationchange: ((this: Window, ev: Event) => any) | null;
 declare var onemptied: ((this: Window, ev: Event) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
 declare var onended: ((this: Window, ev: Event) => any) | null;
-/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-declare var onerror: OnErrorEventHandler;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
 declare var onfocus: ((this: Window, ev: FocusEvent) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */

diff --git a/baselines/ts5.9/dom.generated.d.ts b/baselines/ts5.9/dom.generated.d.ts
@@ -15093,7 +15093,7 @@ interface GlobalEventHandlersEventMap {
     "durationchange": Event;
     "emptied": Event;
     "ended": Event;
-    "error": ErrorEvent;
+    "error": Event;
     "focus": FocusEvent;
     "focusin": FocusEvent;
     "focusout": FocusEvent;
@@ -15234,7 +15234,7 @@ interface GlobalEventHandlers {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
     onended: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-    onerror: OnErrorEventHandler;
+    onerror: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
     onfocus: ((this: GlobalEventHandlers, ev: FocusEvent) => any) | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */
@@ -39457,6 +39457,7 @@ interface WindowEventMap extends GlobalEventHandlersEventMap, WindowEventHandler
     "devicemotion": DeviceMotionEvent;
     "deviceorientation": DeviceOrientationEvent;
     "deviceorientationabsolute": DeviceOrientationEvent;
+    "error": ErrorEvent;
     "gamepadconnected": GamepadEvent;
     "gamepaddisconnected": GamepadEvent;
     "orientationchange": Event;
@@ -39609,6 +39610,8 @@ interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandler
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
      */
     ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+    onerror: OnErrorEventHandler;
     /**
      * @deprecated
      *
@@ -41791,6 +41794,8 @@ declare var ondeviceorientation: ((this: Window, ev: DeviceOrientationEvent) =>
  * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/deviceorientationabsolute_event)
  */
 declare var ondeviceorientationabsolute: ((this: Window, ev: DeviceOrientationEvent) => any) | null;
+/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/error_event) */
+declare var onerror: OnErrorEventHandler;
 /**
  * @deprecated
  *
@@ -42171,8 +42176,6 @@ declare var ondurationchange: ((this: Window, ev: Event) => any) | null;
 declare var onemptied: ((this: Window, ev: Event) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLMediaElement/ended_event) */
 declare var onended: ((this: Window, ev: Event) => any) | null;
-/** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLElement/error_event) */
-declare var onerror: OnErrorEventHandler;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/focus_event) */
 declare var onfocus: ((this: Window, ev: FocusEvent) => any) | null;
 /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLFormElement/formdata_event) */

diff --git a/inputfiles/patches/events.kdl b/inputfiles/patches/events.kdl
@@ -29,7 +29,7 @@ interface-mixin GlobalEventHandlers {
   event dragover type=DragEvent
   event dragstart type=DragEvent
   event drop type=DragEvent
-  event error type=ErrorEvent
+  event error type=Event
   event focus type=FocusEvent
   event focusin type=FocusEvent
   event focusout type=FocusEvent
@@ -76,7 +76,6 @@ interface-mixin GlobalEventHandlers {
   event transitionstart type=TransitionEvent
   event transitionend type=TransitionEvent
   event transitioncancel type=TransitionEvent
-  property onerror overrideType=OnErrorEventHandler
 }
 
 interface-mixin MessageEventTarget {
@@ -109,6 +108,8 @@ interface Window {
   event gamepadconnected type=GamepadEvent
   event gamepaddisconnected type=GamepadEvent
   event DOMContentLoaded type=Event
+  event error type=ErrorEvent
+  property onerror overrideType="OnErrorEventHandler" mdnUrl="https://developer.mozilla.org/docs/Web/API/Window/error_event"
 }
 
 interface WorkerGlobalScope {

diff --git a/src/build/emitter.ts b/src/build/emitter.ts
@@ -856,7 +856,12 @@ export function emitWebIdl(
   // A covariant EventHandler is one that is defined in a parent interface as then redefined in current interface with a more specific argument types
   // These patterns are unsafe, and flagged as error under --strictFunctionTypes.
   // Here we know the property is already defined on the interface, we elide its declaration if the parent has the same handler defined
+  // Exception: if the property has an overrideType, we should emit it as an intentional override
   function isCovariantEventHandler(i: Browser.Interface, p: Browser.Property) {
+    // If this property has an explicit overrideType, emit it (it's an intentional override)
+    if (p.overrideType) {
+      return false;
+    }
     return (
       isEventHandler(p) &&
       iNameToEhParents[i.name].some((parent) =>
@@ -996,13 +1001,20 @@ export function emitWebIdl(
     prefix: string,
     emitScope: EmitScope,
     i: Browser.Interface,
+    emittedProperties?: Set<string>,
   ) {
     if (i.properties) {
       mapToArray(i.properties.property)
         .filter((m) => matchScope(emitScope, m))
         .filter((p) => !isCovariantEventHandler(i, p))
+        .filter((p) => !emittedProperties || !emittedProperties.has(p.name))
         .sort(compareName)
-        .forEach((p) => emitProperty(prefix, i, emitScope, p));
+        .forEach((p) => {
+          emitProperty(prefix, i, emitScope, p);
+          if (emittedProperties) {
+            emittedProperties.add(p.name);
+          }
+        });
     }
   }
 
@@ -1148,11 +1160,12 @@ export function emitWebIdl(
     prefix: string,
     emitScope: EmitScope,
     i: Browser.Interface,
+    emittedProperties?: Set<string>,
   ) {
     const conflictedMembers = extendConflictsBaseTypes[i.name]
       ? extendConflictsBaseTypes[i.name].memberNames
       : new Set<string>();
-    emitProperties(prefix, emitScope, i);
+    emitProperties(prefix, emitScope, i, emittedProperties);
     const methodPrefix = prefix.startsWith("declare var")
       ? "declare function "
       : "";
@@ -1164,13 +1177,21 @@ export function emitWebIdl(
 
   /// Emit all members of every interfaces at the root level.
   /// Called only once on the global polluter object
-  function emitAllMembers(i: Browser.Interface) {
-    emitMembers(/*prefix*/ "declare var ", "InstanceOnly", i);
+  function emitAllMembers(
+    i: Browser.Interface,
+    emittedProperties: Set<string> = new Set(),
+  ) {
+    emitMembers(
+      /*prefix*/ "declare var ",
+      "InstanceOnly",
+      i,
+      emittedProperties,
+    );
 
     for (const relatedIName of iNameToIDependList[i.name]) {
       const i = allInterfacesMap[relatedIName];
       if (i) {
-        emitAllMembers(i);
+        emitAllMembers(i, emittedProperties);
       }
     }
   }

diff --git a/unittests/files/eventlistener.ts b/unittests/files/eventlistener.ts
@@ -1,9 +1,54 @@
-document.addEventListener("arbitrary_invalid_event", (ev) => {
-  return ev.returnValue;
-});
-
 document.addEventListener("arbitrary_invalid_event", {
   handleEvent(ev)  {
     return ev.returnValue;
+  },
+});
+const divElement: HTMLElement = document.createElement("div");
+
+/**
+ * addEventListener works with a single event arg, Event
+ */
+divElement.addEventListener(
+  "click",
+  (event: Event) => {
+    if (event) {
+      return;
+    }
+  },
+  false,
+);
+/**
+ * HTMLElement addEventListener works with a custom string and single event arg, Event
+ */
+divElement.addEventListener("beep", (event: Event) => {
+  if (event) {
+    return;
+  }
+});
+
+const svgElement = document.createElementNS(
+  "http://www.w3.org/2000/svg",
+  "svg",
+);
+
+/**
+ * SVGElement addEventListener works with a single event arg, Event
+ */
+svgElement.addEventListener(
+  "click",
+  (event: Event) => {
+    if (event) {
+      return;
+    }
+  },
+  false,
+);
+
+/**
+ * SVGElement addEventListener works with a custom string and single event arg, Event
+ */
+svgElement.addEventListener("beep", (event: Event) => {
+  if (event) {
+    return;
   }
 });