[toolbar] How to handle vertical toolbars?

Toolbars are commonly horizontal and so this is the default[1]. However, it's inevitable that some will be vertical, we've discussed the orientation attribute as a means to achieve this before. Perhaps defining that is enough and there's nothig more to do.

However, I think it would be good to have on record the thinking here.

• What styles should be used to achieve the vertical UI? (writing-mode perhaps?)
• Should changing styles affect the arrow-key directionality automatically?
• Should that also affect aria-orientation?

I could see us defining a default horizontal behaviour and allowing orientation="vertical" to change that, but this comes against the desire for responsive design where a toolbar might want to become vertical on mobile?

[1] aria-orientation="false" is the default for the role, and inline is a default modifier in focusgroup for the toolbar behaviour.

Start with the linked orientation issue (#1201) and the referenced aria-orientation and focusgroup behavior. Work through how writing-mode, arrow-key directionality, aria-orientation, and responsive changes should interact, and document the project's agreed guidance for vertical toolbars.