Ignore "fake" window unload events in PortRPC

In VitalSource custom/synthetic "unload" events are dispatched at the book's
container frame when switching chapters. Make sure these don't trigger the code
in the `PortRPC` class that is meant to be called when the window is unloaded.

These synthetic events were triggered after every chapter navigation. In Safari
<= 15 the second and subsequent events would cause an error in the
`currentWindow.parent.postMessage` call because the port had already been
transferred after the first event was handled.

src/shared/messaging/port-rpc.ts

@@ -236,7 +236,15 @@ export class PortRPC<OnMethod extends string, CallMethod extends string>
     // send the "close" event through the message channel when the window
     // containing the sending port is unloaded.
     if (!('onclose' in MessagePort.prototype) || forceUnloadListener) {
-      this._listeners.add(currentWindow, 'unload', () => {
+      this._listeners.add(currentWindow, 'unload', event => {
+        // Ignore custom events which use the same name. This works around an
+        // issue in VitalSource.
+        //
+        // See https://github.com/hypothesis/support/issues/161#issuecomment-2454560641.
+        if (event instanceof CustomEvent) {
+          return;
+        }
+
         if (this._port) {
           // Send "close" notification directly. This works in Chrome, Firefox and
           // Safari >= 16.

src/shared/messaging/test/port-rpc-test.js

@@ -260,6 +260,25 @@ describe('PortRPC', () => {
     assert.calledWith(closeHandler);
   });
 
+  // See https://github.com/hypothesis/support/issues/161#issuecomment-2454560641
+  it('ignores "fake" window unload events', async () => {
+    const { port1, port2 } = new MessageChannel();
+    const sender = new PortRPC({ forceUnloadListener: true });
+    const receiver = new PortRPC();
+    const closeHandler = sinon.stub();
+
+    receiver.on('close', closeHandler);
+    receiver.connect(port2);
+    sender.connect(port1);
+    await waitForMessageDelivery();
+
+    assert.notCalled(closeHandler);
+    window.dispatchEvent(new CustomEvent('unload'));
+    await waitForMessageDelivery();
+
+    assert.notCalled(closeHandler);
+  });
+
   it('should send "close" event when MessagePort emits "close" event', async () => {
     const { port1, port2 } = new MessageChannel();
     const sender = new PortRPC();