Function restate_viewport 

pub(crate) fn restate_viewport(app: Entity) -> OnFrame

Restate the scene’s letterboxed region on the pass that draws it.

A window resize changes the region, and the library’s present renders the pass this frame, so this runs before it. The blit pass, when there is one, keeps its full-target viewport. A skipped frame still restates the region: keeping the pass’s viewport in step with the window costs nothing and leaves the pass right for the frame that draws again.