Refs #721.
Reconciled onto the virtualised row list from #799: the selection is keyed on RowAt { path, RowId } rather than the flat list index, since collapsing a file above the selection re-points flat indices.
The overlay built its whole patch as a nested element tree on every frame:
a card per file, a header per hunk, six elements per line. gpui notifies the
view on each scroll wheel event, so a few hundred lines of diff rebuilt tens
of thousands of elements tens of times a second, and the window stalled.
Flatten the tree into one row per line in a new `diff_list` module and draw it
with `gpui::list`, which builds only the rows on screen. The rows are rebuilt
only when what they are built from changes, so scrolling no longer re-splits
hunks or re-clones every line, and a change to one file splices just the rows
it touched rather than resetting the list and losing the scroll position.
The key that decides a rebuild takes the snapshot each frame was asked about
even when it matched only by contents. A probe that finds nothing new still
lands a fresh `Arc` over an equal snapshot; a key left pointing at the old one
would go on walking the whole patch to prove the two equal, once per wheel
event, which is the cost the key exists to avoid.
A list counts a row it has not laid out yet as zero tall, which left the
scrollbar reading an 800-line patch as one viewport: its thumb filled the
track, and a drag from top to bottom travelled 248px and stopped. The rows
below the fold are counted at the 19px both views already give a line of a
patch, through `ListState::with_size_hint` — added to the gpui fork for this,
`Cargo.lock` following its `tty7` branch to `ece710e3`.
A card cannot survive that flattening — its rows are separate items now — so
the frame it drew is gone, and with it the grey header bars and hunk bands
that made the overlay the one view in the app still speaking gpui-component's
default container language. The rows take the source control panel's own
measurements instead: 26px, 10px inset, 5px radius, colour only under the
pointer. The title bar's view switch loses its border for the same reason.
l0ng-ai