event: set this arg on callback

src/netsurf.zig

@@ -622,6 +622,11 @@ pub fn eventTargetAddEventListener(
     const cbk_ptr = try alloc.create(Callback);
     cbk_ptr.* = cbk;
 
+    // When a function is used as an event handler, its this parameter is bound
+    // to the DOM element on which the listener is placed.
+    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this#this_in_dom_event_handlers
+    try cbk_ptr.setThisArg(et);
+
     const ctx = @as(*anyopaque, @ptrCast(cbk_ptr));
     var listener: ?*EventListener = undefined;
     const errLst = c.dom_event_listener_create(event_handler, ctx, &listener);

src/xhr/xhr.zig

@@ -837,4 +837,16 @@ pub fn testExecFn(
         .{ .src = "req4.responseText.length > 64", .ex = "true" },
     };
     try checkCases(js_env, &post);
+
+    var cbk = [_]Case{
+        .{ .src = "const req5 = new XMLHttpRequest()", .ex = "undefined" },
+        .{ .src = "req5.open('GET', 'http://httpbin.io/json')", .ex = "undefined" },
+        .{ .src = "var status = 0; req5.onload = function () { status = this.status };", .ex = "function () { status = this.status }" },
+        .{ .src = "req5.send()", .ex = "undefined" },
+
+        // Each case executed waits for all loop callaback calls.
+        // So the url has been retrieved.
+        .{ .src = "status", .ex = "200" },
+    };
+    try checkCases(js_env, &cbk);
 }