[select] What exactly should happen when tab or shift+tab is pressed on an option in the listbox?

[josepharhar]

Previous keyboard issues here:
#433
#386

I couldn't figure out where the previous discussion on the tab key was, but what we have implemented in chromium is that the popup listbox should close when the tab key is pressed. However, should the next focusable item after the select's button get focused after tab is pressed, or does that not matter? What about shift+tab?

@scottaohara