feat: default diffs to branch changes

docs/configuration.md

@@ -49,6 +49,7 @@ Plugin paths resolve relative to the config file, not the shell's working direct
 `config.diff_view_mode` selects the initial view for each new comparison: `"split"` (default) or `"unified"`. `config.diff_split_orientation` selects `"vertical"` (left/right, default) or `"horizontal"` (above/below) for split views. Invalid values reject the configuration. Normal-mode **Space v** cycles split/unified; **Space s** flips split orientation. These controls affect the current comparison, not startup defaults, and retain their choices across file switches and tabs. Orientation changes apply only in split view.
 
 ```ruby
+config.bind "ctrl+b", config.core.diff.mode.toggle
 config.bind "leader v", config.core.diff.view.toggle
 config.bind "leader s", config.core.diff.orientation.toggle
 ```
@@ -160,7 +161,7 @@ Normal `dd` and `cc` delete/change complete rows, with leading counts. Bind `con
 | Normal | `save`, `force_save`, `quit`, `force_quit`, `restart`, `force_restart`, `save_and_quit`, `force_save_and_quit`, `plugins` |
 | Normal | `edit`, `buffers`, `buffer`, `buffer_next`, `buffer_previous`, `buffer_delete`, `buffer_force_delete` |
 | Normal | `follow_link` |
-| Normal | `diff`, `diff.view.toggle`, `diff.orientation.toggle` |
+| Normal | `diff`, `diff.mode.toggle`, `diff.view.toggle`, `diff.orientation.toggle` |
 | Normal, Insert, Visual, Command, Search | `diff.close` |
 | Normal, Insert, Visual, Command, Search | `word_wrap.toggle` |
 | Normal | `pane.split_horizontal`, `pane.split_vertical`, `pane.focus_left`, `pane.focus_down`, `pane.focus_up`, `pane.focus_right`, `pane.close`, `pane.force_close` |
@@ -237,7 +238,7 @@ config.bind "ctrl+h", nil, mode: config.modes.tree
 
 The diff's changed-file explorer and floating picker use these same tree and selection contexts, including remaps and disabled keys. Enter opens the selected file's comparison; Working checkout After reuses its editable buffer. The catalog contains only changed paths, with A/M/D/R status markers, rather than a disk scan.
 
-Ordinary `config.core.file_picker` and `config.core.file_explorer` bindings work inside comparisons. Ctrl+w closes a focused picker/explorer, retaining its query, selection, and expansion state; the panes use the reclaimed width. Ctrl+e has no reserved diff behavior. Escape closes the revision selector in either stage, but an opened comparison still closes explicitly through `:diff.close`, its palette action, or the generated action value below. `config.core.diff` remains the opening action and exposes immutable `close`, `view.toggle`, and `orientation.toggle` children.
+Ordinary `config.core.file_picker` and `config.core.file_explorer` bindings work inside comparisons. Ctrl+w closes a focused picker/explorer, retaining its query, selection, and expansion state; the panes use the reclaimed width. Ctrl+e has no reserved diff behavior. Escape closes the revision selector in either stage, but an opened comparison still closes explicitly through `:diff.close`, its palette action, or the generated action value below. `config.core.diff` remains the opening action and exposes immutable `close`, `mode.toggle`, `view.toggle`, and `orientation.toggle` children. New comparisons default to Branch changes (merge base → selected After); Normal-mode Ctrl+b, `:diff.mode.toggle`, or the palette toggles to direct Snapshots and back, preserving the selected endpoints. Selecting Working checkout on either side forces Snapshots.
 
 `config.core.pane.close` and `config.core.pane.force_close` close a focused **docked** file list instead of its editor pane, without discarding buffers or exiting the comparison. A floating picker does not intercept these actions: normal close still protects underlying dirty buffers, while force-close retains its existing discard semantics. For example, bind `config.bind "ctrl+x", config.core.pane.close`; focused tree/selection overrides and printable filter input retain priority. Palette-originated close actions retain the originating dock target through force confirmation.
 

docs/editing.md

@@ -313,11 +313,14 @@ Run `:diff` in Normal mode, choose **Before**, then **After**, and press Enter t
 
 The floating selector has **Revisions / Bookmarks** tabs for jj, or **Commits / Branches / Tags** for Git. Type to filter by revision/change ID, commit ID, reference name, or description. **Working checkout** stays first in every category, even when it does not match the query. As Before it is a disk snapshot, including eligible untracked files. As After it includes unsaved regular-text buffers and enables live editing; opening a comparison never saves them.
 
+New comparisons default to **Branch changes**: Before becomes the unique merge base of the selected Before and After revisions, and After stays the selected tip. Changes exclusive to the Before branch are excluded. The Before border identifies both selected names and the immutable merge-base commit ID. **Snapshots** compares the two selected snapshots directly. **Ctrl+b** toggles the mode, also available as **`:diff.mode.toggle`**, the palette action, and remappable **`config.core.diff.mode.toggle`**. Toggling an opened comparison recomputes using the original selected endpoints. Selecting **Working checkout** on either side immediately forces **Snapshots**; Branch changes is unavailable while it is selected. Missing or multiple merge bases produce an explicit, recoverable selector error rather than silently falling back; choose Snapshots to compare such histories. jj's synthetic root is the empty ancestor of every jj history.
+
 | Selector input | Effect |
 | --- | --- |
 | Tab / Shift+Tab | Next / previous category |
 | Up / Down, Ctrl+k / Ctrl+j | Previous / next entry |
 | Backspace / Ctrl+u | Remove a query grapheme / clear query |
+| Ctrl+b | Toggle Branch changes / Snapshots (Working checkout requires Snapshots) |
 | Enter | Choose the entry |
 | Esc | Close either selector stage, cancel outstanding work, and restore the source buffer without discarding edits |