On device: scroll far down a wrapped file, relaunch, and the app lands
further DOWN than where the user left off — the deeper the scroll, the
further off.
Root cause: the persisted Scroll is a pixel offset in VISUAL-line space.
Restoring maps it through the WrapIndex (scrollDecompose -> LineForVisual),
but on relaunch every count is the estimate (1) until the line is shaped,
and shaping covers the visible window only — the lines ABOVE the restored
viewport are never shaped. With all-ones counts LineForVisual maps the
offset 1:1, landing a logical line deeper by every wrapped continuation
above the viewport, and the state is stable (the under-counted lines never
re-enter the window), so it never self-corrects.
The snapshot now persists wrap-independent coordinates: the logical line
at the viewport top (derived with the same mapping the layout uses,
against the current index, so it is exactly the shown line) plus the
sub-line remainder. The restore re-derives the offset as line*lh + sub,
which maps to the saved line under any wrap state (all-ones or populated).
The raw Dp offset is kept for pre-line-coordinate session files
(loadSession defaults the missing key to -1; BeginRestore rejects the
ambiguous zero value: a genuine line-0 snapshot always has Scroll < lh).
TestRestore_ScrollSurvivesWrapState reproduces it: 150 lines recorded as
wrapped x3, viewport at logical line 200 (visual 500); a relaunch with a
fresh WrapIndex must land the window on line 200. Fails pre-fix (window at
line 254, i.e. deeper) and passes with the fix.
Docs: spec §2.4 (line-based scroll persist + current save policy),
architecture §6.7 (why the offset is unrestorable by re-mapping).
Greg Pomerantz
6968f6a284