diff: add a true in-place "inline" overlay (virtual text) as the future layout = "inline"

[ThomasK33]

Summary

Follow-up to #293 / #195. The diff_opts.layout = "unified" option (renamed from "inline" before v0.4.0) renders a unified diff in a separate vsplit pane — a single read-only buffer with interleaved red/green lines. This issue tracks adding a true in-place inline overlay: the diff shown as virtual text layered onto the file in the same window, the way users typically picture a "VS Code-style inline" diff (cf. mini.diff, gitsigns preview, sidekick NES, Cursor inline edits).

When implemented, this mode is the one that should claim layout = "inline" — which is exactly why we reserved that name in #293.

"unified" vs the proposed "inline"

• "unified" (shipped): single read-only buffer in a rightbelow vsplit, deleted (red/strikethrough) + added (green) lines interleaved. Compact, but it's a separate pane and not editable before accept.
• "inline" (this issue): no extra window — proposed changes rendered in place over the real file.

Suggested approach (keeps pre-accept editability)

The unified/read-only path forfeits editing the proposal before accepting. A prototype pattern that keeps it (the codecompanion/sidekick model):

• Proposed/added lines = real, editable buffer lines, highlighted (e.g. ClaudeCodeInlineDiffAdd) with a + sign.
• Deleted lines = virtual text via nvim_buf_set_extmark virt_lines (strikethrough/red), so they're visible but not part of the editable content.
• On accept, the buffer content already is the new file (deleted lines were never real text), so edits made during review are captured. On reject, clear the extmarks / restore.

A pure-Lua, zero-dependency engine (not mini.diff/diffview — see #169) keeps this aligned with the project's philosophy. Bonus: a single-buffer extmark view sets no &diff windows, which sidesteps the closeAllDiffTabs foreign-diff class (#277) and the stale-diff repaint issues (#205).

Notes

• The current "unified" path doesn't fire the ClaudeCodeDiffOpened / ClaudeCodeDiffClosed User autocmds that the native diff does — worth wiring up for any new mode.
• @wookayin expressed interest ("once we implement that") on feat(diff): add layout = "inline" for VS Code-style unified diff #195 — collaboration welcome.

Refs #293, #195, #82

🤖 Generated with Claude Code

[ThomasK33]

Note

This triage report is AI-generated using Mux

Triage report

Classification: feature request.

This is a valid follow-up to the current layout = "unified" work rather than a bug in the shipped unified layout. The requested behavior is a new in-place review mode: proposed content is shown in the original file window without opening an extra diff pane, while deleted old content is rendered as virtual lines/extmarks.

What I verified

• The current configuration accepts diff_opts.layout = "vertical", "horizontal", or "unified"; "inline" is intentionally rejected right now.
• The existing "unified" path dispatches to lua/claudecode/diff_inline.lua, creates a scratch buffer, opens it with rightbelow vsplit, renders additions/deletions in that buffer, and sets it non-modifiable. That matches the issue's description of the current behavior and confirms that a true in-place overlay mode does not exist yet.
• The requested design is meaningfully different from the current unified split: to preserve pre-accept editability, added/proposed lines should become real editable buffer lines, while deleted lines should be virtual-only display state such as virt_lines extmarks.
• The zero-dependency direction fits the existing implementation style: the current unified path already uses pure Lua plus Neovim APIs rather than adding mini.diff/diffview as dependencies.

Recommendation

Keep this open and implement it as a new layout = "inline" feature, while preserving layout = "unified" for the existing single-buffer split view. A good implementation slice would include:

1. Extend config/types/docs/tests to accept diff_opts.layout = "inline" with wording that clearly distinguishes it from "unified".
2. Add a separate in-place engine instead of mutating the current read-only unified split path too much.
3. For existing-file edits, apply the proposed buffer content into the real target buffer for review, mark added/proposed regions with highlights/signs, and render deleted old lines with virtual lines/extmarks.
4. On accept, return the current reviewed buffer content so user edits made during review are preserved.
5. On reject/cleanup, restore the original file content, clear extmarks/signs/autocmds, and avoid leaving modified or hidden buffers behind.
6. Ensure the inline mode emits the same diff lifecycle events as the native layouts.
7. Keep foreign-diff safety explicit: the new mode should not set &diff or call diffthis, so it should avoid the class of issues where Claude diffs interact with diffview/vimdiff tabs.

Suggested acceptance criteria

• layout = "inline" opens no extra diff window for existing-file edits.
• Deleted lines are visible but not part of the editable buffer content.
• Added/proposed lines are editable before accept.
• Accept preserves any review-time edits made in the buffer.
• Reject restores the original content and removes all inline diff UI state.
• New-file, delete-only, mixed-change, and no-trailing-newline cases are covered by tests.
• ClaudeCodeDiffOpened / ClaudeCodeDiffClosed fire for the inline layout with useful window/buffer metadata.
• Existing vertical, horizontal, and unified layouts keep their current behavior.

Adjacent finding

While checking the lifecycle-event requirement, I found that the current layout = "unified" path appears to skip the documented ClaudeCodeDiffOpened / ClaudeCodeDiffClosed events. I created #296 as a separate, narrower bug so this feature request can stay focused on the in-place overlay.

Validation performed

• Inspected lua/claudecode/config.lua, lua/claudecode/types.lua, lua/claudecode/diff.lua, lua/claudecode/diff_inline.lua, README.md, and the diff-related unit specs.
• Reviewed related history: [FEATURE] Inline diff #82, feat(diff): add layout = "inline" for VS Code-style unified diff #195, diff: reconsider layout = "inline" naming before v0.4.0 (reserve "inline" for a true in-place overlay) #293, [BUG] closeAllDiffTabs closes all diff-mode windows, destroying unrelated diffs (diffview.nvim) #277, and [BUG] Diff panels not cleared when new suggestion is shown — stale diffs accumulate until window focus change #205.
• Searched existing open/closed issues before creating bug(diff): unified layout skips diff lifecycle User events #296 to avoid a duplicate adjacent bug.
• Ran mise exec -- busted -v tests/unit/diff_inline_spec.lua tests/unit/diff_auto_resize_events_spec.lua — 39 successes, 0 failures.
• Ran mise run check — 0 warnings, 0 errors.