Improve inline spinner performance (#5592)

As a followup to #5571, improve performance for inline spinners too.

This requires a bit of preparation; in particular, we change
ListContextTrait.HandleRender to no longer force a UI layout/redraw, so
callers need to take care of this themselves where needed. This requires
careful testing to see that we didn't miss any situation where redrawing
only worked by accident; see the first commit of the branch for an
example.