feat: inline (unified) diff layout with treesitter syntax highlighting (#281)

## Summary

Add a new inline diff layout as an alternative to side-by-side. Deleted
lines appear as virtual line overlays with treesitter syntax
highlighting, providing a VSCode-style unified diff experience.

Set `diff.layout = "inline"` to enable. Default remains `"side-by-side"`
— no breaking changes.

## Features

- **Inline diff rendering** — single-window layout with deleted lines as
`virt_lines` overlays
- **Treesitter syntax highlighting** on virtual lines — deleted code
retains keyword, string, function colors merged with diff backgrounds
- **Character-level diff highlights** on both virtual lines (deleted)
and real lines (inserted)
- **Full mode support** — works with standalone, explorer, and history
modes
- **Shared keymaps** — `do` reverts hunk to original, `dp` is no-op
(buffer already has modified content), navigation/textobject/staging all
work
- **44 new tests** covering all inline scenarios with 0 regressions

## Changes

### New files
- `lua/codediff/ui/inline.lua` — core renderer (virt_lines, char
highlights, treesitter syntax)
- `lua/codediff/ui/view/inline_view.lua` — inline engine
(create/update/rerender/show_single_file)
- `tests/ui/inline_spec.lua` — 10 renderer unit tests
- `tests/ui/inline_syntax_spec.lua` — 8 treesitter syntax tests
- `tests/ui/view/inline_standalone_spec.lua` — 14 standalone view tests
- `tests/ui/view/inline_explorer_spec.lua` — 4 explorer mode tests
- `tests/ui/view/inline_history_spec.lua` — 4 history mode tests
- `tests/ui/view/inline_interaction_spec.lua` — 12 interaction tests
(nav, do, autoscroll, textobject, lifecycle)

### Modified files (additive, gated by `layout == "inline"`)
- `config.lua` — `layout = "side-by-side"` option
- `view/init.lua` — router dispatches to inline engine
- `keymaps.lua` — layout-aware do/dp/find_hunk_at_cursor
- `navigation.lua` — inline uses modified line numbers
- `auto_refresh.lua` — inline render branch
- `lifecycle/cleanup.lua` — threshold for inline sessions
- `lifecycle/state.lua` — inline namespace clear + resume render
- `explorer/render.lua` — A/D/?? routing to inline_view.show_single_file
- `history/render.lua` — A/D routing to inline_view.show_single_file

## Usage

```lua
require('codediff').setup({
  diff = { layout = 'inline' },
})
```

## Testing
- 44 new inline-specific tests
- All existing tests pass (5 made layout-resilient)
- Full suite passes with both `inline` and `side-by-side` as default

Author: esmuellert

README.md

@@ -2,7 +2,7 @@
 
 [![Downloads](https://img.shields.io/github/downloads/esmuellert/vscode-diff.nvim/total?label=⬇%20downloads&color=blue)](https://github.com/esmuellert/vscode-diff.nvim/releases)
 
-A Neovim plugin that provides VSCode-style side-by-side diff rendering with two-tier highlighting.
+A Neovim plugin that provides VSCode-style diff rendering with two-tier highlighting, supporting both side-by-side and inline (unified) layouts.
 
 <div align="center">
 
@@ -24,6 +24,7 @@ https://github.com/user-attachments/assets/64c41f01-dffe-4318-bce4-16eec8de356e
   - Light backgrounds for entire modified lines (green for insertions, red for deletions)
   - Deep/dark character-level highlights showing exact changes within lines
 - **Side-by-side diff view** in a new tab with synchronized scrolling
+- **Inline (unified) diff view** — single-window layout with deleted lines as virtual overlays, with treesitter syntax highlighting
 - **Git integration**: Compare between any git revision (HEAD, commits, branches, tags)
 - **Same implementation as VSCode's diff engine**, providing identical visual highlighting for most scenarios
 - **Fast C-based diff computation** using FFI with **multi-core parallelization** (OpenMP)
@@ -82,6 +83,7 @@ https://github.com/user-attachments/assets/64c41f01-dffe-4318-bce4-16eec8de356e
 
     -- Diff view behavior
     diff = {
+      layout = "side-by-side",             -- Diff layout: "side-by-side" (two panes) or "inline" (single pane with virtual lines)
       disable_inlay_hints = true,         -- Disable inlay hints in diff windows for cleaner view
       max_computation_time_ms = 5000,     -- Maximum time for diff computation (VSCode default)
       ignore_trim_whitespace = false,     -- Ignore leading/trailing whitespace changes (like diffopt+=iwhite)

VERSION

@@ -1 +1 @@
-2.38.0
+2.39.0

doc/codediff.txt

@@ -1,7 +1,7 @@
 codediff.nvim                                 *codediff.txt* *codediff.nvim* *codediff*
 
-CodeDiff provides VSCode-style side-by-side diff rendering with two-tier
-highlighting.
+CodeDiff provides VSCode-style diff rendering with two-tier highlighting,
+supporting both side-by-side and inline (unified) layouts.
 
 This help file is usage-focused. For installation, build, and development
 details, see README.md.
@@ -114,6 +114,22 @@ Git merge tool:
 ==============================================================================
 CONFIGURATION                                        *codediff-configuration*
 
+                                                      *codediff-layout*
+Diff layout~
+
+CodeDiff supports two diff layouts:
+
+  `"side-by-side"` (default): Two panes with synchronized scrolling.
+  `"inline"`: Single pane with deleted lines shown as virtual overlays.
+              Includes treesitter syntax highlighting on virtual lines.
+
+Set the layout in your config:
+>lua
+  require("codediff").setup({
+    diff = { layout = "inline" },
+  })
+<
+
 Setup entry point:
 >lua
   require("codediff").setup({
@@ -129,6 +145,7 @@ Setup entry point:
       conflict_sign_rejected = nil,
     },
     diff = {
+      layout = "side-by-side",
       disable_inlay_hints = true,
       max_computation_time_ms = 5000,
       hide_merge_artifacts = false,

doc/tags

@@ -8,6 +8,7 @@ codediff-events	codediff.txt	/*codediff-events*
 codediff-highlight-groups	codediff.txt	/*codediff-highlight-groups*
 codediff-history	codediff.txt	/*codediff-history*
 codediff-history-line-range	codediff.txt	/*codediff-history-line-range*
+codediff-layout	codediff.txt	/*codediff-layout*
 codediff-lua-api	codediff.txt	/*codediff-lua-api*
 codediff-quickstart	codediff.txt	/*codediff-quickstart*
 codediff-see-also	codediff.txt	/*codediff-see-also*

lua/codediff/config.lua

@@ -29,6 +29,7 @@ M.defaults = {
 
   -- Diff view behavior
   diff = {
+    layout = "inline", -- Diff layout: "side-by-side" or "inline"
     disable_inlay_hints = true, -- Disable inlay hints in diff windows for cleaner view
     max_computation_time_ms = 5000, -- Maximum time for diff computation (5 seconds, VSCode default)
     ignore_trim_whitespace = false, -- Ignore leading/trailing whitespace changes (like diffopt+=iwhite)

lua/codediff/ui/auto_refresh.lua

@@ -99,7 +99,15 @@ local function do_diff_update(bufnr, skip_watcher_check)
     -- Update stored diff result in lifecycle (critical for hunk navigation and do/dp)
     lifecycle.update_diff_result(tabpage, lines_diff)
 
-    -- Update decorations on both buffers
+    -- Check if this is an inline mode session
+    local session = lifecycle.get_session(tabpage)
+    if session and session.layout == "inline" then
+      local inline_mod = require("codediff.ui.inline")
+      inline_mod.render_inline_diff(modified_bufnr, lines_diff, original_lines, modified_lines)
+      return
+    end
+
+    -- Side-by-side mode: Update decorations on both buffers
     core.render_diff(original_bufnr, modified_bufnr, original_lines, modified_lines, lines_diff)
 
     -- Re-sync scrollbind after filler changes

lua/codediff/ui/explorer/render.lua

@@ -188,51 +188,66 @@ function M.create(status_result, git_root, tabpage, width, base_revision, target
     -- Handle untracked files: show file without diff
     if file_data.status == "??" then
       vim.schedule(function()
-        local side_by_side = require("codediff.ui.view.side_by_side")
-        side_by_side.show_untracked_file(tabpage, abs_path)
+        local sess = lifecycle.get_session(tabpage)
+        if sess and sess.layout == "inline" then
+          require("codediff.ui.view.inline_view").show_single_file(tabpage, abs_path)
+        else
+          require("codediff.ui.view.side_by_side").show_untracked_file(tabpage, abs_path)
+        end
       end)
       return
     end
 
     -- Handle added files: only one side has the file
     if file_data.status == "A" then
-      if base_revision and target_revision and target_revision ~= "WORKING" then
-        -- Two-revision mode: show target revision content only
-        vim.schedule(function()
-          local side_by_side = require("codediff.ui.view.side_by_side")
-          side_by_side.show_added_virtual_file(tabpage, git_root, file_path, target_revision)
-        end)
-      elseif group == "staged" then
-        -- Staged mode: file exists in staging (:0) but not in HEAD
-        vim.schedule(function()
-          local side_by_side = require("codediff.ui.view.side_by_side")
-          side_by_side.show_added_virtual_file(tabpage, git_root, file_path, ":0")
-        end)
-      else
-        -- Unstaged/working tree: file exists on disk but not in any revision
-        vim.schedule(function()
-          local side_by_side = require("codediff.ui.view.side_by_side")
-          side_by_side.show_untracked_file(tabpage, abs_path)
-        end)
-      end
+      vim.schedule(function()
+        local sess = lifecycle.get_session(tabpage)
+        local is_inline = sess and sess.layout == "inline"
+
+        if base_revision and target_revision and target_revision ~= "WORKING" then
+          if is_inline then
+            require("codediff.ui.view.inline_view").show_single_file(tabpage, file_path, { revision = target_revision, git_root = git_root, rel_path = file_path })
+          else
+            require("codediff.ui.view.side_by_side").show_added_virtual_file(tabpage, git_root, file_path, target_revision)
+          end
+        elseif group == "staged" then
+          if is_inline then
+            require("codediff.ui.view.inline_view").show_single_file(tabpage, file_path, { revision = ":0", git_root = git_root, rel_path = file_path })
+          else
+            require("codediff.ui.view.side_by_side").show_added_virtual_file(tabpage, git_root, file_path, ":0")
+          end
+        else
+          if is_inline then
+            require("codediff.ui.view.inline_view").show_single_file(tabpage, abs_path)
+          else
+            require("codediff.ui.view.side_by_side").show_untracked_file(tabpage, abs_path)
+          end
+        end
+      end)
       return
     end
 
     -- Handle deleted files: show old content without diff
     if file_data.status == "D" then
-      if base_revision and target_revision and target_revision ~= "WORKING" then
-        -- Two-revision mode: use base_revision
-        vim.schedule(function()
-          local side_by_side = require("codediff.ui.view.side_by_side")
-          side_by_side.show_deleted_virtual_file(tabpage, git_root, file_path, base_revision)
-        end)
-      else
-        -- Working tree mode: use staged or HEAD
-        vim.schedule(function()
-          local side_by_side = require("codediff.ui.view.side_by_side")
-          side_by_side.show_deleted_file(tabpage, git_root, file_path, abs_path, group)
-        end)
-      end
+      vim.schedule(function()
+        local sess = lifecycle.get_session(tabpage)
+        local is_inline = sess and sess.layout == "inline"
+
+        if base_revision and target_revision and target_revision ~= "WORKING" then
+          if is_inline then
+            require("codediff.ui.view.inline_view").show_single_file(tabpage, file_path, { revision = base_revision, git_root = git_root, rel_path = file_path })
+          else
+            require("codediff.ui.view.side_by_side").show_deleted_virtual_file(tabpage, git_root, file_path, base_revision)
+          end
+        else
+          if is_inline then
+            local revision = (group == "staged") and "HEAD" or ":0"
+            require("codediff.ui.view.inline_view").show_single_file(tabpage, file_path, { revision = revision, git_root = git_root, rel_path = file_path })
+          else
+            require("codediff.ui.view.side_by_side").show_deleted_file(tabpage, git_root, file_path, abs_path, group)
+          end
+        end
+      end)
       return
     end
 

lua/codediff/ui/history/render.lua

@@ -294,16 +294,22 @@ function M.create(commits, git_root, tabpage, width, opts)
     end
 
     vim.schedule(function()
-      -- Handle added/deleted files: show single pane instead of empty diff
+      -- Handle added/deleted files: show single file instead of empty diff
       local file_status = file_data.status
       if file_status == "A" or file_status == "D" then
-        local side_by_side = require("codediff.ui.view.side_by_side")
-        if file_status == "A" then
-          -- File added in this commit: only exists in commit_hash
-          side_by_side.show_added_virtual_file(tabpage, git_root, file_path, commit_hash)
+        local sess = lifecycle.get_session(tabpage)
+        local is_inline = sess and sess.layout == "inline"
+
+        if is_inline then
+          local rev = file_status == "A" and commit_hash or target_hash
+          local path = file_status == "D" and (old_path or file_path) or file_path
+          require("codediff.ui.view.inline_view").show_single_file(tabpage, path, { revision = rev, git_root = git_root, rel_path = path })
         else
-          -- File deleted in this commit: only exists in parent
-          side_by_side.show_deleted_virtual_file(tabpage, git_root, old_path or file_path, target_hash)
+          if file_status == "A" then
+            require("codediff.ui.view.side_by_side").show_added_virtual_file(tabpage, git_root, file_path, commit_hash)
+          else
+            require("codediff.ui.view.side_by_side").show_deleted_virtual_file(tabpage, git_root, old_path or file_path, target_hash)
+          end
         end
         return
       end