DiffTextView.ReloadFile ran DiffAlignment.Build synchronously for both the
constructor and the File-property path, so a large diff (or several at once
in Planning mode's per-file ItemsControl) could freeze the UI thread.
Diffs at or under 500 raw lines still build inline (no placeholder frame);
larger ones build via Task.Run and a generation counter discards the result
if File changed again before the build finished.
Mika Kuns
c9afc81f0a