Comboboxes are not working at all in WebKitGtk+, neither for WK1 not WK2. The popup window is not displayed, and the combobox does not even work with the keyboard.
I don't think this is specific to GTK. The problem is that r147548 made the popup to be closed when a blur event is received. Right before showing the popup the input element is focused, so that after the the popup is shown the focus event is emitted and the event handler is called again hiding the popup. I guess this doesn't happen in other platforms because they run a nested mainloop for popups. A possible solution would be to check that the current element has actually lost the focus before hiding the popup.
Created attachment 201695 [details]
Committed r150102: <http://trac.webkit.org/changeset/150102>