follow_selection=false stored scroll as a raw rendered-line offset into
lines rebuilt fresh every frame. A ProjectionUpdate::Patch growing a
streaming row above the viewport shifted every line below it, so the
raw offset kept pointing at a numerically-same but visually different
line and the viewport drifted even though the operator had scrolled
away deliberately.
Cache each frame's row-id-per-line layout and, before applying scroll
next frame, translate the old raw offset into (row id, intra-row line)
against that cache, then relocate it in the freshly rebuilt lines. Rows
above or intersecting the viewport growing, shrinking, or being
patched now shift the offset instead of the visible content. Falls
back to the nearest surviving neighbor row when the anchor row itself
is removed, and to the clamped raw offset when nothing nearby
survived. follow_selection and bottom-follow are unaffected — they
already recompute scroll from the live selection/tail each frame.
Miguel Palhas
6ae7c3eb97