webapi: fix UIEvent.view default, initCustomEvent guard, KeyboardEvent flags

Fixes 4 failing tests in WPT /dom/events/Event-init-while-dispatching.html
(1/5 -> 5/5), and as a side effect most of
Event-subclasses-constructors.html (16/49 -> 46/49). Three distinct
bugs, all about synthetic (constructor-created) events not following
the spec's dictionary defaults:

- UIEvent.view defaulted to the window: the constructor stored
  `opts.view orelse frame.window` and the getter fell back to the
  window too, so `new UIEvent("x").view` was never null. Per
  UIEventInit, view defaults to null. The window fallback is kept for
  *trusted* (browser-generated) UI events only, which do target the
  window, so internal dispatch call sites stay unchanged.
- initCustomEvent was missing the "short-circuit while dispatching"
  guard that initEvent and the other legacy initializers have.
- KeyboardEvent's constructor unconditionally forced bubbles,
  cancelable and composed to true (correct only for trusted key events
  per UI Events); synthetic KeyboardEvents now follow the EventInit
  defaults.

The unit tests asserting the old view-defaults-to-window behavior are
updated to expect null.

Coverage: /dom/events/Event-init-while-dispatching.html 1/5 -> 5/5,
/dom/events/Event-subclasses-constructors.html 16/49 -> 46/49. No
regressions across /dom/events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/browser/tests/event/composition.html

@@ -11,7 +11,8 @@
     testing.expectEqual("test", event.type);
     testing.expectEqual("", event.data);
     testing.expectEqual(0, event.detail);
-    testing.expectEqual(window, event.view);
+    // per UIEventInit, view defaults to null for synthetic events
+    testing.expectEqual(null, event.view);
   }
 </script>
 

src/browser/tests/event/ui.html

@@ -11,7 +11,7 @@
     testing.expectEqual(5, evt.detail);
     testing.expectEqual(true, evt.bubbles);
     testing.expectEqual(true, evt.cancelable);
-    testing.expectEqual(window, evt.view);
+    testing.expectEqual(null, evt.view);
 </script>
 
 <script id=uiEventWithView>
@@ -30,7 +30,7 @@
     testing.expectEqual(0, evt3.detail);
     testing.expectEqual(false, evt3.bubbles);
     testing.expectEqual(false, evt3.cancelable);
-    testing.expectEqual(window, evt3.view);
+    testing.expectEqual(null, evt3.view);
 </script>
 
 <script id=uiEventInheritance>

src/browser/webapi/event/CustomEvent.zig

@@ -66,6 +66,11 @@ pub fn initCustomEvent(
     cancelable: ?bool,
     detail_: ?js.Value.Global,
 ) !void {
+    // Like initEvent, a no-op while the event is being dispatched.
+    if (self._proto._event_phase != .none) {
+        return;
+    }
+
     // This function can only be called after the constructor has called.
     // So we assume proto is initialized already by constructor.
     self._proto._type_string = try String.init(self._proto._arena, event_string, .{});

src/browser/webapi/event/KeyboardEvent.zig

@@ -321,11 +321,15 @@ fn initWithTrusted(arena: Allocator, typ: String, _opts: ?Options, trusted: bool
 
     Event.populatePrototypes(event, opts, trusted);
 
-    // https://w3c.github.io/uievents/#event-type-keyup
-    const rootevt = event._proto._proto;
-    rootevt._bubbles = true;
-    rootevt._cancelable = true;
-    rootevt._composed = true;
+    if (trusted) {
+        // Browser-generated key events bubble, are cancelable and composed:
+        // https://w3c.github.io/uievents/#event-type-keyup
+        // Synthetic ones follow the EventInit dictionary defaults.
+        const rootevt = event._proto._proto;
+        rootevt._bubbles = true;
+        rootevt._cancelable = true;
+        rootevt._composed = true;
+    }
 
     return event;
 }

src/browser/webapi/event/UIEvent.zig

@@ -66,7 +66,7 @@ pub fn init(typ: []const u8, _opts: ?Options, frame: *Frame) !*UIEvent {
             ._type = .generic,
             ._proto = undefined,
             ._detail = opts.detail,
-            ._view = opts.view orelse frame.window,
+            ._view = opts.view,
         },
     );
 
@@ -109,8 +109,16 @@ pub fn getDetail(self: *UIEvent) u32 {
 
 // sourceCapabilities not implemented
 
-pub fn getView(self: *UIEvent, frame: *Frame) *Window {
-    return self._view orelse frame.window;
+pub fn getView(self: *UIEvent, frame: *Frame) ?*Window {
+    if (self._view) |view| {
+        return view;
+    }
+    // Trusted (browser-generated) UI events implicitly target the window;
+    // synthetic events default to a null view per the UIEventInit dictionary.
+    if (self._proto._is_trusted) {
+        return frame.window;
+    }
+    return null;
 }
 
 // Legacy: see https://w3c.github.io/uievents/#dom-uievent-which