how to close combobox when onmouseout?

Answer 1

You can't do this (reliably). The popup that appears is not available to manipulate, ie open or close, programmatically and its behaviour is defined by the browser (or operating system).

To expand on this further, both IE and Firefox can close the popup by blurring the select element:

selectEl.blur();

Although, the mouseout event fires even when you move the mouse to the options in the popup, so it would require a bit of hackery magic. In Chrome it will blur the select element but the box will remain open.

It's generally best to leave the behaviour of UI components alone, so that users get the experience they expect through interaction with your website.