fix for the beforeunload dialog in the proxyless mode (#2833)

src/client/sandbox/event/unload.ts

@@ -77,6 +77,8 @@ export default class UnloadSandbox extends SandboxBase {
     }
 
     private _createEventHandler (eventProperties: EventProperties): Function {
+        const that = this;
+
         return function (e, originListener): void {
             // NOTE: Overriding the returnValue property to prevent a native dialog.
             nativeMethods.objectDefineProperty(e, 'returnValue', createPropertyDesc({
@@ -107,6 +109,13 @@ export default class UnloadSandbox extends SandboxBase {
                 eventProperties.storedReturnValue = UnloadSandbox._prepareStoredReturnValue(res);
                 eventProperties.prevented         = true;
             }
+
+            // NOTE: need to return value to show `beforeunload` dialog
+            // in proxyless mode to handle the dialog in the cdp
+            if (that.proxyless)
+                return res;
+
+            return void 0;
         };
     }