Clip the research draft to its pane and return to the reader's place

A draft whose anchor is partly scrolled out no longer draws over the
document tabs. The opening reveal goes back to the scroll offset the
reader had when the draft opened if it fits there, and jumps instead of
gliding when the trip is longer than the pane. The new e2e places its
caret by clicking, since End scrolls the document on macOS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>