Toggling a CollapsingHeader in a ComboBox closes the whole ComboBox

**Describe the bug**
Putting a CollapsingHeader inside a ComboBox closes the ComboBox whenever you try to toggle the CollapsingHeader

**To Reproduce**
Steps to reproduce the behavior:
1. Create a ComboBox
2. Put a CollapsingHeader inside
3. Toggle the CollapsingHeader

```rust
egui::ComboBox::from_label("Version")
.width(150.0)
.selected_text(selected_version)
.show_ui(ui, |ui| {
egui::CollapsingHeader::new("Dev")
.default_open(true)
.show(ui, |ui| {
for version in versions.iter() {
ui.selectable_value(&mut selected_version, &version, *version);
}
});
});
````

**Expected behavior**
The CollapsingHeader should be able to open/close without the ComboBox closing

**Screenshots**

**System**
OS: Windows10 20H2