v0.57.0

Fixed

• Scrolling a long diff (for example a single generated file with tens of
  thousands of lines) stays smooth instead of becoming clunky. The Diff panel
  used to rescan the whole diff every frame, both to find the first visible line
  and to measure the widest line for the horizontal-scroll clamp. Both are now
  cached by content, so a frame costs the same at the bottom of a huge diff as at
  the top, and a flood of scroll events no longer saturates the event loop (which
  was also stalling the file-list refresh). Measured on a 1.7 MB diff, draining a
  burst of scroll events dropped from about 1.6 s to about 0.5 s. The same cached
  index speeds up scrolling in the staging view and the commit-graph viewer.
• The Diff panel keeps its scroll position when you focus it to read and then
  move back to the list. It used to snap to the top every time focus crossed in
  or out of the panel, even though the same diff was still shown; it now resets
  only when the shown content actually changes (a different file, commit, or
  branch).
• Leaving the W diffing view no longer drops the Diff panel at a stale scroll
  position. After scrolling a long ref diff, exiting (or entering) diffing now
  resets the panel to the top, instead of opening the next preview clamped to a
  seemingly random offset.
• A slow or very large diff preview no longer freezes the file list. The
  background working-tree refresh used to pause while any preview was loading, so
  a heavy preview (for example diffing a branch against another ref in a large
  repository) could stall it and stop the Files panel from picking up changes.
  The refresh now runs independently of previews. The branch, commit-range, and
  directory diffs also load a bounded summary first and skip the full patch when
  it is enormous, the guard the single-commit preview already used.

Full Changelog: v0.56.0...v0.57.0