For example, a browser-native <select> today will be able to expand beyond the borders of the hosting browser window, so you can don't have to worry about it getting clipped to the window borders, but a fancy emulated select from toolkit like Quasar will not, ,which limits its placement options. So to do this right on win32 you would need each popover to have its own native HWND and on Cocoa/macOS you would need it in its own NSView.
Does anybody in here know if this is how it is actually implemented in the current browsers, or is it just a paint-over job inside the browser window?