feat: show line change counts in diff tabs

docs/configuration.md

@@ -140,7 +140,7 @@ Builtins are `:tabs`, `:battery`, `:mode`, `:path`, `:position`, `:language`, `:
 
 A bar with no visible content consumes no row. For example, `config.bar.top = { left: [], center: [], right: [] }` removes the top bar. A bar whose only visible content is `:tabs` is hidden with a single tab, whether above or below the workspace. Narrow layouts first omit inactive tabs, then optional segments from group ends to avoid overlap; the active tab drops decorations before its title is clipped. Commands and buffer searches use floating input; operation feedback uses a bottom-right popup. Neither reserves a footer row.
 
-Each segment's `title` is a literal template supporting only `{index}`, `{view}`, `{name}`, `{path}`, and `{modified}`: the one-based tab index, comparison-view marker, filename, file path, and dirty marker (`" [+]"` when dirty, `""` when clean). `{view}` expands to `"Diff · "` when the tab owns a comparison or revision selector, otherwise `""`. Tabs use each tab's view and buffer metadata; other segments use the active context. Tabs default to `"{index} {view}{name}{modified}"`; custom templates without `{view}` retain their output. An empty title on another segment uses its builtin content or named label value. Templates are not Ruby code and never invoke a render callback.
+Each segment's `title` is a literal template supporting only `{index}`, `{view}`, `{name}`, `{path}`, and `{modified}`: the one-based tab index, comparison-view marker, filename, file path, and dirty marker (`" [+]"` when dirty, `""` when clean). `{view}` expands to `"Diff · +N -M · "` for a loaded comparison file, counting added and removed lines; revision selectors and files still loading use `"Diff · "`, and ordinary tabs use `""`. Counts follow the selected file and live edits, including inactive tabs. Tabs use each tab's view and buffer metadata; other segments use the active context. Tabs default to `"{index} {view}{name}{modified}"`; custom templates without `{view}` retain their output. An empty title on another segment uses its builtin content or named label value. Templates are not Ruby code and never invoke a render callback.
 
 `:path` and `{path}` abbreviate the actual home-directory prefix to `~/` (`~` for the home directory itself). This is display-only: save paths, buffer identity, and LSP file URIs retain the original path. Sibling directories with similar names are not abbreviated.
 

docs/editing.md

@@ -427,6 +427,8 @@ Comparisons start in **split, vertical (left/right)** view by default. In Normal
 
 Diff metadata lives in the text-pane borders instead of a separate info row: split panes show their own Before/After revision labels at the bottom-left; a single visible pane shows the combined `before → after` label there. Each pane shows its read-only/editable state at the top-right, independently of keyboard focus. Before is always read-only; After is editable only for an eligible Working-checkout or checkout-matched pinned file. The changed-file explorer and panes start directly below configured chrome, and there is no native-buffer help footer. Loading/error/file notices remain visible when applicable. Narrow borders prioritize state and clip titles without painting over the gutter or source text.
 
+Default diff tab titles show `Diff · +N -M · filename`, with added and removed line counts for the selected file, not the whole comparison. Replaced lines count on both sides; unchanged context does not count. Counts update with live edits and undo/redo, including when the diff tab is inactive. Revision selectors and files still loading omit counts until their text diff is ready. Custom chrome templates include this prefix through `{view}`.
+
 For jj-managed repositories, revision entries and border labels display only the change ID; Git repositories keep commit-ID labels, and named bookmarks retain their names. The backing commit ID remains the exact comparison target and can still be used to filter revisions.
 
 Text panes use normal buffer interaction regardless of writability: motions and counts, search, Visual/Visual Line selections, multiple carets, yank, remapped bindings, and non-mutating actions work in Before and After. Read-only snapshots reject insert/delete/change/paste, repeat edits, undo/redo, mutating plugin effects, and saves, including saves to another path. Their text retains the revision's final newline. Snapshots use separate virtual LSP document identities and do not appear in ordinary buffer lists or dirty-buffer comparisons. Closing the diff disposes unreferenced snapshots and restores the live or original buffer in its workspace pane; snapshots displayed by other panes or tabs remain read-only until closed. Explicit buffer commands and Markdown/plugin navigation retain their ordinary behavior; native LSP definitions use the routing below.