Bug 1688089 - Add use counters for 'overflow'/'underflow' event listeners. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D102646

dom/base/UseCounters.conf

@@ -78,6 +78,10 @@ custom onstart sets a <marquee> onstart event listener
 custom onbounce sets a <marquee> onbounce event listener
 custom onfinish sets a <marquee> onfinish event listener
 
+// Element non-standard events
+custom onoverflow sets an element onoverflow event listener
+custom onunderflow sets an element onunderflow event listener
+
 // JavaScript feature usage
 custom JS_asmjs uses asm.js
 custom JS_wasm uses WebAssembly

dom/events/EventListenerManager.cpp

@@ -401,6 +401,18 @@ void EventListenerManager::AddEventListenerInternal(
     if (nsPIDOMWindowInner* window = GetInnerWindowForTarget()) {
       window->SetHasBeforeInputEventListenersForTelemetry();
     }
+  } else if (aTypeAtom == nsGkAtoms::onoverflow) {
+    if (nsPIDOMWindowInner* window = GetInnerWindowForTarget()) {
+      if (Document* doc = window->GetExtantDoc()) {
+        doc->SetUseCounter(eUseCounter_custom_onoverflow);
+      }
+    }
+  } else if (aTypeAtom == nsGkAtoms::onunderflow) {
+    if (nsPIDOMWindowInner* window = GetInnerWindowForTarget()) {
+      if (Document* doc = window->GetExtantDoc()) {
+        doc->SetUseCounter(eUseCounter_custom_onunderflow);
+      }
+    }
   }
 
   if (IsApzAwareListener(listener)) {