When keyboard focus is inside any of the icon toolbars and the mouse cursor is on one of its buttons, it's not possible to navigate between the toolbar buttons with arrow keys. The focus moves at first but immediately bounces back to the button under the mouse.
Unfortunately I see no way how we could change anything about this behaviour, that's just how WinAPI toolbars appear to work (I think the focus only changes after the button's tooltips disappears and this is what interferes with the keyboard focus).
