[FIX] slots: make sure handlers are properly bound to component

In some situations (a slot inside a slot), the combine method was
wrongly copying all properties of the scope into the context, which
caused the event handling system to wrongly use a subobject as component
(since it detects the fact that __owl__ is a own property_).
Consequently, we could have very subtle issue with some properties being
shadowed by a sub object.

src/qweb/qweb.ts

@@ -139,8 +139,9 @@ const UTILS: Utils = {
   },
   combine(context, scope) {
     const clone = Object.create(context);
-    for (let k in scope) {
-      clone[k] = scope[k];
+    while (!isComponent(scope)) {
+      Object.assign(clone, scope);
+      scope = scope.__proto__;
     }
     return clone;
   },

tests/component/slots.test.ts

@@ -1257,4 +1257,50 @@ describe("t-slot directive", () => {
       `<div><div class="slotted"><div class="slot"><div class="child"></div></div></div></div>`
     );
   });
+
+  test("t-slot scope context", async () => {
+    expect.assertions(1);
+
+    class Wrapper extends Component {
+      static template = xml`<t t-slot="default"/>`;
+    }
+
+    let dialog;
+
+    class Dialog extends Component {
+      static template = xml`
+        <Wrapper>
+          <div t-on-click="onClick">
+            <t t-slot="default" />
+          </div>
+        </Wrapper>
+      `;
+
+      static components = { Wrapper };
+
+      setup() {
+        dialog = this;
+      }
+
+      onClick(ev) {
+        // we do not use expect(this).toBe(dialog) here because if it fails, it
+        // may blow up jest because it then tries to compute a diff, which is
+        // infinite if there is a cycle
+        expect(this === dialog).toBe(true);
+      }
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <Dialog>
+            <button>The Button</button>
+        </Dialog>`;
+      static components = { Dialog };
+    }
+
+    await mount(Parent, { target: fixture });
+
+    document.querySelector("button").click();
+    await nextTick();
+  });
 });