fix(commands): add diff buffer UX improvements

Problem: diffs:// buffers could trigger spurious LSP diagnostics,
opening multiple diffs from fugitive created redundant splits, and
there was no quick way to close diff windows.

Solution: disable diagnostics on diff buffers, reuse existing
diffs:// windows in the tabpage instead of creating new splits,
and add a buffer-local q keymap to close diff windows.

lua/diffs/commands.lua

@@ -3,6 +3,27 @@ local M = {}
 local git = require('diffs.git')
 local dbg = require('diffs.log').dbg
 
+---@return integer?
+function M.find_diffs_window()
+  local tabpage = vim.api.nvim_get_current_tabpage()
+  for _, win in ipairs(vim.api.nvim_tabpage_list_wins(tabpage)) do
+    if vim.api.nvim_win_is_valid(win) then
+      local buf = vim.api.nvim_win_get_buf(win)
+      local name = vim.api.nvim_buf_get_name(buf)
+      if name:match('^diffs://') then
+        return win
+      end
+    end
+  end
+  return nil
+end
+
+---@param bufnr integer
+function M.setup_diff_buf(bufnr)
+  vim.diagnostic.enable(false, { bufnr = bufnr })
+  vim.keymap.set('n', 'q', '<cmd>close<CR>', { buffer = bufnr })
+end
+
 ---@param diff_lines string[]
 ---@param hunk_position { hunk_header: string, offset: integer }
 ---@return integer?
@@ -96,9 +117,16 @@ function M.gdiff(revision, vertical)
     vim.api.nvim_buf_set_var(diff_buf, 'diffs_repo_root', repo_root)
   end
 
-  vim.cmd(vertical and 'vsplit' or 'split')
+  local existing_win = M.find_diffs_window()
-  vim.api.nvim_win_set_buf(0, diff_buf)
+  if existing_win then
+    vim.api.nvim_set_current_win(existing_win)
+    vim.api.nvim_win_set_buf(existing_win, diff_buf)
+  else
+    vim.cmd(vertical and 'vsplit' or 'split')
+    vim.api.nvim_win_set_buf(0, diff_buf)
+  end
 
+  M.setup_diff_buf(diff_buf)
   dbg('opened diff buffer %d for %s against %s', diff_buf, rel_path, revision)
 
   vim.schedule(function()
@@ -190,8 +218,14 @@ function M.gdiff_file(filepath, opts)
     vim.api.nvim_buf_set_var(diff_buf, 'diffs_old_filepath', old_rel_path)
   end
 
-  vim.cmd(opts.vertical and 'vsplit' or 'split')
+  local existing_win = M.find_diffs_window()
-  vim.api.nvim_win_set_buf(0, diff_buf)
+  if existing_win then
+    vim.api.nvim_set_current_win(existing_win)
+    vim.api.nvim_win_set_buf(existing_win, diff_buf)
+  else
+    vim.cmd(opts.vertical and 'vsplit' or 'split')
+    vim.api.nvim_win_set_buf(0, diff_buf)
+  end
 
   if opts.hunk_position then
     local target_line = M.find_hunk_line(diff_lines, opts.hunk_position)
@@ -201,6 +235,7 @@ function M.gdiff_file(filepath, opts)
     end
   end
 
+  M.setup_diff_buf(diff_buf)
   dbg('opened diff buffer %d for %s (%s)', diff_buf, rel_path, diff_label)
 
   vim.schedule(function()
@@ -244,9 +279,16 @@ function M.gdiff_section(repo_root, opts)
   vim.api.nvim_buf_set_name(diff_buf, 'diffs://' .. diff_label .. ':all')
   vim.api.nvim_buf_set_var(diff_buf, 'diffs_repo_root', repo_root)
 
-  vim.cmd(opts.vertical and 'vsplit' or 'split')
+  local existing_win = M.find_diffs_window()
-  vim.api.nvim_win_set_buf(0, diff_buf)
+  if existing_win then
+    vim.api.nvim_set_current_win(existing_win)
+    vim.api.nvim_win_set_buf(existing_win, diff_buf)
+  else
+    vim.cmd(opts.vertical and 'vsplit' or 'split')
+    vim.api.nvim_win_set_buf(0, diff_buf)
+  end
 
+  M.setup_diff_buf(diff_buf)
   dbg('opened section diff buffer %d (%s)', diff_buf, diff_label)
 
   vim.schedule(function()

spec/ux_spec.lua

@@ -0,0 +1,135 @@
+local commands = require('diffs.commands')
+local helpers = require('spec.helpers')
+
+local counter = 0
+
+local function create_diffs_buffer(name)
+  counter = counter + 1
+  local bufnr = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, {
+    'diff --git a/file.lua b/file.lua',
+    '--- a/file.lua',
+    '+++ b/file.lua',
+    '@@ -1,1 +1,2 @@',
+    ' local x = 1',
+    '+local y = 2',
+  })
+  vim.api.nvim_set_option_value('buftype', 'nowrite', { buf = bufnr })
+  vim.api.nvim_set_option_value('bufhidden', 'wipe', { buf = bufnr })
+  vim.api.nvim_set_option_value('swapfile', false, { buf = bufnr })
+  vim.api.nvim_set_option_value('modifiable', false, { buf = bufnr })
+  vim.api.nvim_set_option_value('filetype', 'diff', { buf = bufnr })
+  vim.api.nvim_buf_set_name(bufnr, name or ('diffs://unstaged:file_' .. counter .. '.lua'))
+  return bufnr
+end
+
+describe('ux', function()
+  describe('diagnostics', function()
+    it('disables diagnostics on diff buffers', function()
+      local bufnr = create_diffs_buffer()
+      commands.setup_diff_buf(bufnr)
+
+      assert.is_false(vim.diagnostic.is_enabled({ bufnr = bufnr }))
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+    end)
+
+    it('does not affect other buffers', function()
+      local diff_buf = create_diffs_buffer()
+      local normal_buf = helpers.create_buffer({ 'hello' })
+
+      commands.setup_diff_buf(diff_buf)
+
+      assert.is_true(vim.diagnostic.is_enabled({ bufnr = normal_buf }))
+      vim.api.nvim_buf_delete(diff_buf, { force = true })
+      helpers.delete_buffer(normal_buf)
+    end)
+  end)
+
+  describe('q keymap', function()
+    it('sets q keymap on diff buffer', function()
+      local bufnr = create_diffs_buffer()
+      commands.setup_diff_buf(bufnr)
+
+      local keymaps = vim.api.nvim_buf_get_keymap(bufnr, 'n')
+      local has_q = false
+      for _, km in ipairs(keymaps) do
+        if km.lhs == 'q' then
+          has_q = true
+          break
+        end
+      end
+      assert.is_true(has_q)
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+    end)
+
+    it('q closes the window', function()
+      local bufnr = create_diffs_buffer()
+      commands.setup_diff_buf(bufnr)
+
+      vim.cmd('split')
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(win, bufnr)
+
+      local win_count_before = #vim.api.nvim_tabpage_list_wins(0)
+
+      vim.api.nvim_buf_call(bufnr, function()
+        vim.cmd('normal q')
+      end)
+
+      local win_count_after = #vim.api.nvim_tabpage_list_wins(0)
+      assert.equals(win_count_before - 1, win_count_after)
+    end)
+  end)
+
+  describe('window reuse', function()
+    it('returns nil when no diffs window exists', function()
+      local win = commands.find_diffs_window()
+      assert.is_nil(win)
+    end)
+
+    it('finds existing diffs:// window', function()
+      local bufnr = create_diffs_buffer()
+      vim.cmd('split')
+      local expected_win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(expected_win, bufnr)
+
+      local found = commands.find_diffs_window()
+      assert.equals(expected_win, found)
+
+      vim.api.nvim_win_close(expected_win, true)
+    end)
+
+    it('ignores non-diffs buffers', function()
+      local normal_buf = helpers.create_buffer({ 'hello' })
+      vim.cmd('split')
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(win, normal_buf)
+
+      local found = commands.find_diffs_window()
+      assert.is_nil(found)
+
+      vim.api.nvim_win_close(win, true)
+      helpers.delete_buffer(normal_buf)
+    end)
+
+    it('returns first diffs window when multiple exist', function()
+      local buf1 = create_diffs_buffer()
+      local buf2 = create_diffs_buffer()
+
+      vim.cmd('split')
+      local win1 = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(win1, buf1)
+
+      vim.cmd('split')
+      local win2 = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_buf(win2, buf2)
+
+      local found = commands.find_diffs_window()
+      assert.is_not_nil(found)
+      assert.is_true(found == win1 or found == win2)
+
+      vim.api.nvim_win_close(win1, true)
+      vim.api.nvim_win_close(win2, true)
+    end)
+  end)
+end)