…dows (#277)

closeAllDiffTabs previously closed every window with `&diff` set and
force-deleted any buffer named like `*.diff`/`diff://`/`fugitive://`,
without checking that claudecode created them. The Claude CLI invokes
this tool at the start of every user turn while an IDE is connected, so
an open diffview.nvim, fugitive, or native `:diffsplit` review was wiped
out on essentially every prompt — only the diffview file panel (not a
`&diff` window) survived, matching the reported symptom.

Scope the tool to claudecode's own tracked-diff registry only (drop the
window sweep and the buffer force-delete). This mirrors the official VS
Code extension, which closes only the diff tabs it labelled
"[Claude Code] ..." — an ownership check, not an all-diffs sweep.

Also guard the second defect: `find_main_editor_window` (in both
open_file.lua and diff.lua) did not exclude windows in diff mode, so
openFile/openDiff could `:edit` into one half of a foreign diff, which
clears that window's `diff` option and drops its partner out of the diff
group. Skip `&diff` windows in both finders, the open_file fallback, and
the buffer-reuse branch of the diff setup code.

Adds a reproduction fixture (fixtures/issue-277, diffview.nvim) and an
agent-tty + MCP harness (scripts/repro_issue_277.sh); both defects flip
from 3/3 reproduced to 0/3 with this change. Rewrites the two unit tests
that pinned the old destructive sweep and adds &diff coverage to the
window-finder specs.

Change-Id: If023bdcbcd1bcd10b6169282112ce75e09e17c56
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Kosiewski <tk@coder.com>

…diff windows (#277)

Review follow-up. The &diff guard added to find_main_editor_window stops
openDiff from :edit-ing INTO a foreign diff window, but when the current
tab consists ENTIRELY of foreign diff windows (vimdiff/diffview.nvim/
fugitive) the finder returns nil and `_setup_blocking_diff` fell through
to the issue #231 same-tab split fallback. A tab has a single shared diff
set, so the subsequent `diffthis` made Claude's proposal join — and thus
corrupt — the user's existing diff, the same class of breakage #277 is
about.

Detect that case (current_tab_has_diff_window) and route the diff through
the existing new-tab path (display_terminal_in_new_tab + created_new_tab
cleanup) instead of splitting the foreign diff tab. The terminal-only
fallback (#231) is unchanged: a tab with no diff window still splits in
place.

Verified in real Neovim: openDiff into an all-diff tab now opens a new
tab and leaves the user's 2-way diff intact (2 diff windows, untouched).
Adds tests/unit/diff_all_diff_tab_spec.lua and nvim_tabpage_list_wins to
the vim mock.

Change-Id: I5fe6c0700689cf79cfc216a9e7c90860bc17e0a9
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Kosiewski <tk@coder.com>

…lready has a diff (#277)

Generalizes the previous follow-up. Isolating Claude's diff only when the
WHOLE tab was diff windows missed the case where the tab holds a user
diff plus a normal editor split: find_main_editor_window() returns that
normal window, so target_window is set and the all-diff fallback never
ran -- yet `diffthis` still joins the tab's shared diff set and corrupts
the user's diff (Neovim diffs are tab-scoped).

Check the destination tab (the chosen window's tabpage, or the current
tab when none was found) and route to a dedicated tab whenever it already
contains any foreign &diff window, regardless of whether a usable non-diff
window exists there. The terminal-only #231 split fallback now runs only
when the tab has no diff to corrupt.

Verified in real Neovim: with a user vimdiff + a normal split in one tab,
openDiff used to leave that tab with 4 diff windows (Claude's panes joined
the user's diff); it now opens a new tab and the user's diff stays at 2
windows. Generalizes diff_all_diff_tab_spec.lua (adds the mixed diff+editor
tab case) and replaces _current_tab_has_diff_window with the tabpage-scoped
_tabpage_has_diff_window.

Change-Id: Iffcb9361d963023db51ab2200a644545b7ef0bb0
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Kosiewski <tk@coder.com>

…ver joins a diff (#277)

Review follow-up. In the all-diff-window fallback, openFile :vsplits off a
diff window; Neovim copies window-local options on split, so the new
window inherits `diff`. For most files the subsequent `:edit` clears diff
(entering a non-diff buffer), so the user's diff was left intact -- but
when the opened file is ITSELF one of the diffed buffers, the window stays
in diff mode and shows up as an extra pane in the user's diff set.

Run `:diffoff` (current window only) on the new split before editing so
the opened file can never participate in the user's diff, closing the last
edge of the #277 invariant: openFile/openDiff never disturb a foreign diff.

Verified in real Neovim against the real open_file handler: opening a file
that is part of an active 2-way vimdiff used to leave 3 diff windows; it
now leaves the user's diff at 2 and opens the file in a separate non-diff
window. open_file_spec asserts the diffoff precedes the edit.

Change-Id: I6edfb9744c1cca0cd8fd191c8a5d8abf728cf574
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Thomas Kosiewski <tk@coder.com>