Pressing down arrow when menu containing richcobmo is focused, will not move focus to first potion

[Marek Lewandowski]

until master

I feel that there is a case when down arrow with buttons does not work as expected. It works good with i.e. language button, color buttons - but still does not work as expected with richcombos, because it still does not focus first option.

The exception here is format menu, which checks currently active format.

1. open any sample with CKEditor (i.e. samples/replacebyclass.html)
2. navigate using keyboard to styles dropdown (focus it)
3. press down arrow

Expected result:
First entry in dropdown should be focused, in this case Italic Title.

Current result:
Only menu is focused.

additional info:

1. funny thing is that we don't block down arrow in this case, if scrollbar is visible (in document contianing editor), and its moveable towards bottom, then pressing down arrow will move screen

origin issue:
​t/11075