Another example: default macOS dropdowns allow you to click and drag while holding the button to select something in one single gesture. Default <select> implementation in Chrome/Vivaldi also supports it. None of the custom JS ones support it.
The menu that opens from a <select> extends outside of the browser viewport which makes it easier to choose from long lists.
Anything that opens a custom popup menu, including a dropdown, can only close it when you click within the browser viewport, for obvious reasons. System popup menus (NSMenu I think?) are kinda system-modal in that you can close them by clicking anywhere on screen, which is more convenient.
Your last point is not true. You can set custom dropdowns to close on loss of focus, which will happen when clicking outside of the browser window. It makes debugging with the browaer tools a right pain in the neck, but it's totally possible.
Another example: default macOS dropdowns allow you to click and drag while holding the button to select something in one single gesture. Default <select> implementation in Chrome/Vivaldi also supports it. None of the custom JS ones support it.
The menu that opens from a <select> extends outside of the browser viewport which makes it easier to choose from long lists.
Anything that opens a custom popup menu, including a dropdown, can only close it when you click within the browser viewport, for obvious reasons. System popup menus (NSMenu I think?) are kinda system-modal in that you can close them by clicking anywhere on screen, which is more convenient.