We should add a progress bar component to indicate background loading
Eventually we will probably want to support a progress level (i.e. controlling how much of the bar is filled) but the immediate use case (T285607: Create and use a proper "loading" progress bar vue display) is just for an indeterminate state, where the bar will move across the screen and then jump to the start
Discussed with @Volker_E at https://www.mediawiki.org/wiki/Topic:Wcf9k59gp78enllk
Existing components
Wikimedia community
OOUI
ProgressBarWidget
Implementation notes
Accessibility notes
Needs to feature role="progressbar", and aria-valuemin="0" aria-valuemax="100". aria-valuenow should be omitted for the indeterminate type. See
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_progressbar_role