Do not attempt to render events for streams that are not scrolled out of screen

[abey79]

Querying and rendering the event view for streams in the time panel is, despite recent chunkification, quite slow. This is made worse by the "expand all" behaviour of the filter feature (#8654).

A likely optimisation is:

• to avoid rendering evens for everything that is above the visible part of the scroll area
• early out as soon as we're bellow the visible part of the scroll area