Pan Zoom demo widgets are layered incorrectly

**Describe the bug**


![image](https://github.com/emilk/egui/assets/43298013/92d64bb0-dc09-4fe5-b380-195a7a9404b1)

**Desktop (please complete the following information):**
 - OS: Windows 11, Fedora 39
 - Browser: Firefox