The changes view gains a lens of All, Interface, Implementation, and Tests
that spans the file tree above it, a switch that hides test files from the
tree and the diff, and a viewed checkbox on each file header that collapses
the file. The toolbar counts viewed files and V toggles the file being read.
Marks are kept per Session, source, path, and change digest in browser
storage, so a file that changes again reads as changed since viewed. All
commits beside Submit review, and beside the base branch of the Branch scope,
shows one commit or a Shift run of commits.