Steps to reproduce:

1. Start simplebrowser
2. Navigate to http://kiev.tm/select.html
3. Open combobox
4. Close application

I couldn't reproduce the issue with quick browser.

Actual behavior:

App doesn't exit but hangs. The following error message occurs on Windows:

External WM_DESTROY received for QWidgetWindow(0x8e227e0, name="QtWebEngineCore::RenderWidgetHostViewQtDelegateWidgetClassWindow") , parent: QWindow(0x0) , transient parent: QWindow(0x0)
No context current during QPlatformBackingStore destruction, OpenGL resources not released

There is no error message on Linux.

Expected behavior:

App exits.

I think this issue might be related or similar to https://bugreports.qt.io/browse/QTBUG-54836