fix: adjust jj adapter integration for safety

Reorder adapter probing so GitAdapter is tried before JjAdapter,
avoiding unnecessary `jj root` calls in plain git repos.

Revert generic file.lua changes (bufload/checktime, is_loaded→is_valid)
and replace them with a jj-specific on_local_buffer_reused() hook, so
existing Git/Hg behaviour is unchanged.

Author: dlyongemallo

lua/diffview/tests/functional/jj_adapter_spec.lua

@@ -160,6 +160,32 @@ describe("diffview.vcs.adapters.jj", function()
     end)
   end)
 
+  describe("on_local_buffer_reused()", function()
+    it("calls checktime on an unmodified buffer", function()
+      local adapter = new_adapter()
+      local bufnr = vim.api.nvim_create_buf(true, false)
+
+      -- checktime requires a file on disk; write a temp file so the buffer
+      -- has a real name and checktime doesn't error.
+      local tmpfile = vim.fn.tempname()
+      vim.fn.writefile({ "hello" }, tmpfile)
+      vim.api.nvim_buf_set_name(bufnr, tmpfile)
+      vim.fn.bufload(bufnr)
+
+      -- Should not error.
+      assert.has_no.errors(function()
+        adapter:on_local_buffer_reused(bufnr)
+      end)
+
+      -- Buffer should still be loaded and valid.
+      assert.is_true(vim.api.nvim_buf_is_valid(bufnr))
+      assert.is_true(vim.api.nvim_buf_is_loaded(bufnr))
+
+      pcall(vim.api.nvim_buf_delete, bufnr, { force = true })
+      vim.fn.delete(tmpfile)
+    end)
+  end)
+
   describe("rev_to_args()", function()
     it("returns --from/--to for commit ranges", function()
       local adapter = new_adapter()

lua/diffview/vcs/adapter.lua

@@ -272,6 +272,14 @@ function VCSAdapter:force_entry_refresh_on_noop(left, right)
   return false
 end
 
+---Called when `_create_local_buffer` reuses an existing buffer. Adapters
+---whose VCS rewrites working-copy files (e.g. jj) should override this to
+---ensure the buffer content reflects the current state on disk.
+---@param bufnr integer
+function VCSAdapter:on_local_buffer_reused(bufnr)
+  -- Default: no-op. Git and Hg do not rewrite working-copy files.
+end
+
 ---Restore a file to the requested state
 ---@param path string # file to restore
 ---@param kind '"staged"'|'"working"'

lua/diffview/vcs/adapters/jj/init.lua

@@ -434,6 +434,21 @@ function JjAdapter:force_entry_refresh_on_noop(left, right)
   return self:has_local(left, right)
 end
 
+---Jj may rewrite working-copy files when revisions change, so reload the
+---buffer from disk when it is reused.
+---@param bufnr integer
+function JjAdapter:on_local_buffer_reused(bufnr)
+  local api = vim.api
+
+  if not api.nvim_buf_is_loaded(bufnr) then
+    vim.fn.bufload(bufnr)
+  end
+
+  if not vim.bo[bufnr].modified then
+    pcall(vim.cmd, ("checktime %d"):format(bufnr))
+  end
+end
+
 ---@param left Rev
 ---@param right Rev
 ---@return string[]

lua/diffview/vcs/file.lua

@@ -182,15 +182,8 @@ function File:_create_local_buffer()
   else
     -- NOTE: LSP servers might load buffers in the background and unlist
     -- them. Explicitly set the buffer as listed when loading it here.
-    if not api.nvim_buf_is_loaded(self.bufnr) then
-      vim.fn.bufload(self.bufnr)
-    end
-
-    if not vim.bo[self.bufnr].modified then
-      pcall(vim.cmd, ("checktime %d"):format(self.bufnr))
-    end
-
     vim.bo[self.bufnr].buflisted = true
+    self.adapter:on_local_buffer_reused(self.bufnr)
   end
 
   self:post_buf_created()
@@ -508,7 +501,7 @@ function File:detach_buffer()
 end
 
 function File:dispose_buffer()
-  if self.bufnr and api.nvim_buf_is_valid(self.bufnr) then
+  if self.bufnr and api.nvim_buf_is_loaded(self.bufnr) then
     self:detach_buffer()
 
     if not lib.is_buf_in_use(self.bufnr, { self }) then
@@ -520,14 +513,12 @@ function File:dispose_buffer()
 end
 
 function File.safe_delete_buf(bufnr)
-  if not bufnr or bufnr == File.NULL_FILE.bufnr or not api.nvim_buf_is_valid(bufnr) then
+  if not bufnr or bufnr == File.NULL_FILE.bufnr or not api.nvim_buf_is_loaded(bufnr) then
     return
   end
 
-  if api.nvim_buf_is_loaded(bufnr) then
-    for _, winid in ipairs(utils.win_find_buf(bufnr, 0)) do
-      File.load_null_buffer(winid)
-    end
+  for _, winid in ipairs(utils.win_find_buf(bufnr, 0)) do
+    File.load_null_buffer(winid)
   end
 
   pcall(api.nvim_buf_delete, bufnr, { force = true })

lua/diffview/vcs/init.lua

@@ -17,7 +17,7 @@ local M = {}
 ---@return string? err
 ---@return VCSAdapter? adapter
 function M.get_adapter(opt)
-  local adapter_kinds = { JjAdapter, GitAdapter, HgAdapter, P4Adapter }
+  local adapter_kinds = { GitAdapter, JjAdapter, HgAdapter, P4Adapter }
 
   if not opt.cmd_ctx then
     opt.cmd_ctx = {}