For the non-vue version we use OO.ui.ProgressBarWidget (see [1] under "Progress bar" for a demo)
For the vue version, we currently have a placeholder that just says "LOADING" in the center
We can either wait for WVUI to add a progress bar component, or add one to the GlobalWatchlist repo (and then if one gets added to wvui switch to using that)
I opted to do it directly in wvui, see T286685: Add a progress bar component to WVUI - we can use the new component once a new version of the library is released and made available in ResourceLoader