fix(diff): reuse tabnew window for unified new-tab diff

[zakissimo]

Summary

When the unified diff opens in a new tab (diff_opts = { layout = "unified", open_in_new_tab = true }), the tab contained an extra empty [No Name] pane next to the diff, and — with hide_terminal_in_new_tab = true — the diff's tab could also leak on cleanup. Both stem from diff_inline.setup_inline_diff.

Root cause

1. Empty pane. :tabnew (via display_terminal_in_new_tab) creates the tab with an ephemeral [No Name] window. setup_inline_diff then picks that window as editor_win and does rightbelow vsplit to place the diff beside it, so the empty [No Name] window lingers as a blank pane and wincmd = gives it equal width.

2. Dropped new-tab handle. guard_inline_setup captured the helper's return values with:

local results = { pcall(action) }
...
return unpack(results, 2)

Under hide_terminal_in_new_tab = true, display_terminal_in_new_tab returns nil for the terminal window in the middle of its result list (original_tab, nil, had_terminal, new_tab). That nil is a table hole, so #results reports a border before it and unpack(results, 2) truncates — silently dropping the trailing new-tab handle. Result: new_tab_number is nil, so the tab isn't closed in cleanup_inline_diff, and ClaudeCodeDiffOpened fires with tab_number = nil (breaking the documented "own the layout" hook for this path).

Fix

• Reuse the ephemeral :tabnew window as the diff window when created_new_tab is true (its buffer is bufhidden=wipe, so it self-cleans). The in-current-tab path keeps the rightbelow vsplit so it never replaces the user's own buffer.
• Add a small pack_results helper that records the exact argument count, and switch the guard to unpack(results, 2, results.n) so nils in the middle no longer truncate the returns.

Reproduction

require("claudecode").setup({
  diff_opts = { layout = "unified", open_in_new_tab = true, hide_terminal_in_new_tab = true },
})

Have Claude Code propose an edit. Before: the diff tab shows an empty pane on the left. After: the diff spans the tab (terminal shown → diff + terminal, no empty pane).

Testing

• New tests/unit/diff_inline_new_tab_spec.lua asserts the new tab has exactly the expected windows (no empty pane) for hidden/shown terminal, and that new_tab_number is tracked (not nil). It fails on the pre-fix code and passes after.
• Existing diff_inline_spec.lua and diff_hide_terminal_new_tab_spec.lua still pass.
• Verified end-to-end in real headless Neovim (0.12.3): hidden → 1 full-width diff window; shown → diff + terminal, no empty pane.

🤖 Generated with Claude Code