Auto merge of #22933 - pngai:implement-returnValue-attribute, r=jdm

Implement historical returnValue attribute for Event interface

<!-- Please describe your changes on the following line: -->
Implementation of `returnValue` attribute for Event interface.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #22881 (GitHub issue number if applicable)

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22933)
<!-- Reviewable:end -->

2 files changed, 13 insertions, 0 deletions

diff --git a/components/script/dom/event.rs b/components/script/dom/event.rs
index fc18036bfc7..914cf279278 100644
--- a/components/script/dom/event.rs
+++ b/components/script/dom/event.rs
@@ -297,6 +297,18 @@ impl EventMethods for Event {
         self.cancelable.get()
     }
 
+    // https://dom.spec.whatwg.org/#dom-event-returnvalue
+    fn ReturnValue(&self) -> bool {
+        self.canceled.get() == EventDefault::Allowed
+    }
+
+    // https://dom.spec.whatwg.org/#dom-event-returnvalue
+    fn SetReturnValue(&self, val: bool) {
+        if !val {
+            self.PreventDefault();
+        }
+    }
+
     // https://dom.spec.whatwg.org/#dom-event-timestamp
     fn TimeStamp(&self) -> u64 {
         self.timestamp
diff --git a/components/script/dom/webidls/Event.webidl b/components/script/dom/webidls/Event.webidl
index 6c7861ecc6e..1a6fd8e4a5c 100644
--- a/components/script/dom/webidls/Event.webidl
+++ b/components/script/dom/webidls/Event.webidl
@@ -26,6 +26,7 @@ interface Event {
   readonly attribute boolean bubbles;
   [Pure]
   readonly attribute boolean cancelable;
+  attribute boolean returnValue;  // historical
   void preventDefault();
   [Pure]
   readonly attribute boolean defaultPrevented;