Outline view built every slide's title and text editors each frame, so a 2000-slide deck took
about 40 ms a frame in a release build (more with real text). Slides outside the view (with a
margin) now take their last measured height, estimated from their line count until first
shown; the current slide is always laid out so its editor keeps the focus.
Co-authored-by: Cursor <cursoragent@cursor.com>
