refactor(jj): tidy single-change diff labelling

lua/diffview/api/views/diff/diff_view.lua

@@ -7,7 +7,6 @@ require("diffview.bootstrap")
 local DiffView = lazy.access("diffview.scene.views.diff.diff_view", "DiffView") ---@type DiffView|LazyModule
 local File = lazy.access("diffview.vcs.file", "File") ---@type vcs.File|LazyModule
 local FileEntry = lazy.access("diffview.scene.file_entry", "FileEntry") ---@type FileEntry|LazyModule
-local FilePanel = lazy.access("diffview.scene.views.diff.file_panel", "FilePanel") ---@type FilePanel|LazyModule
 local Rev = lazy.access("diffview.vcs.adapters.git.rev", "GitRev") ---@type GitRev|LazyModule
 local RevType = lazy.access("diffview.vcs.rev", "RevType") ---@type RevType|LazyModule
 local vcs_utils = lazy.require("diffview.vcs") ---@module "diffview.vcs"
@@ -18,14 +17,6 @@ local logger = DiffviewGlobal.logger
 
 local M = {}
 
-local function rev_to_panel_name(adapter, rev_arg, left, right)
-  if adapter.rev_to_panel_name then
-    return adapter:rev_to_panel_name(rev_arg, left, right)
-  end
-
-  return rev_arg or adapter:rev_to_pretty_string(left, right)
-end
-
 ---@class FileData
 ---@field path string Path relative to git root.
 ---@field oldpath string|nil If the file has been renamed, this should be the old path, oterhwise nil.
@@ -69,15 +60,7 @@ function CDiffView:init(opt)
   self.fetch_files = opt.update_files
   self.get_file_data = opt.get_file_data
 
-  self:super(vim.tbl_extend("force", opt, {
-    adapter = adapter,
-    panel = FilePanel(
-      adapter,
-      self.files,
-      self.path_args,
-      rev_to_panel_name(adapter, self.rev_arg, opt.left, opt.right)
-    ),
-  }))
+  self:super(vim.tbl_extend("force", opt, { adapter = adapter }))
 
   if type(opt.files) == "table" and not vim.tbl_isempty(opt.files) then
     local files = self:create_file_entries(opt.files)

lua/diffview/scene/views/diff/diff_view.lua

@@ -31,14 +31,6 @@ local M = {}
 
 local same_rev = lazy.access(rev_lib, "same_rev") --[[@as fun(a: Rev?, b: Rev?): boolean ]]
 
-local function rev_to_panel_name(adapter, rev_arg, left, right)
-  if adapter.rev_to_panel_name then
-    return adapter:rev_to_panel_name(rev_arg, left, right)
-  end
-
-  return rev_arg or adapter:rev_to_pretty_string(left, right)
-end
-
 ---@class DiffViewOptions
 ---@field show_untracked? boolean
 ---@field selected_file? string Path to the preferred initially selected file.
@@ -84,7 +76,7 @@ function DiffView:init(opt)
       self.adapter,
       self.files,
       self.path_args,
-      rev_to_panel_name(self.adapter, self.rev_arg, self.left, self.right)
+      self.adapter:rev_to_panel_name(self.rev_arg, self.left, self.right)
     ),
   })
 
@@ -407,7 +399,7 @@ function DiffView:set_revs(new_rev_arg, opts)
   self.rev_arg = new_rev_arg
   self.left = new_left
   self.right = new_right
-  self.panel.rev_pretty_name = rev_to_panel_name(self.adapter, new_rev_arg, self.left, self.right)
+  self.panel.rev_pretty_name = self.adapter:rev_to_panel_name(new_rev_arg, self.left, self.right)
 
   -- Migrate selection persistence to the new scope key.
   if self._selection_scope_key then
@@ -732,8 +724,13 @@ local update_files_impl = debounce.debounce_trailing(
       perf:lap("updated head rev")
     end
 
-    self.panel.rev_pretty_name =
-      rev_to_panel_name(self.adapter, self.rev_arg, self.left, self.right)
+    -- Recompute the panel label unconditionally: adapter-resolved metadata
+    -- (e.g., a jj change description) can shift even when `left`/`right`
+    -- are structurally unchanged, e.g., `jj describe` on `@` rewrites the
+    -- description but leaves the LOCAL rev's `object_name()` fixed at
+    -- "UNKNOWN", so `refresh_revs` returns nil for that case. Placed after
+    -- the head-rev update so a HEAD move is reflected in the label too.
+    self.panel.rev_pretty_name = self.adapter:rev_to_panel_name(self.rev_arg, self.left, self.right)
     perf:lap("updated rev label")
 
     local index_stat = pl:stat(pl:join(self.adapter.ctx.dir, "index"))

lua/diffview/tests/functional/jj_adapter_spec.lua

@@ -627,46 +627,93 @@ describe("diffview.vcs.adapters.jj", function()
           "update file",
           adapter:rev_to_panel_name(left:abbrev() .. ".." .. right:abbrev(), left, right)
         )
+      end)
 
-        local non_parent = adapter.Rev(RevType.COMMIT, string.rep("1", 40))
-        eq("main..@", adapter:rev_to_panel_name("main..@", non_parent, right))
+      it("substitutes the description for a LOCAL right (`:DiffviewOpen`)", function()
+        if not jj_available() then
+          pending("jj not installed")
+          return
+        end
+
+        repo.write("file.txt", "first\n")
+        repo.jj({ "describe", "-m", "initial" })
+        repo.jj({ "new" })
+        repo.write("file.txt", "second\n")
+        repo.jj({ "describe", "-m", "working copy change" })
+
+        local adapter = repo.adapter()
+        local parent_hash =
+          run({ "jj", "show", "-T", "parents.first().commit_id()", "@", "--no-patch" }, repo.dir)
+        local left = adapter.Rev(RevType.COMMIT, parent_hash)
+        local right = adapter.Rev(RevType.LOCAL)
+
+        eq("working copy change", adapter:rev_to_panel_name(nil, left, right))
       end)
 
-      it("uses the current change description for a default no-arg diff", function()
+      it("includes the description for an initial-commit diff (root..@)", function()
         if not jj_available() then
           pending("jj not installed")
           return
         end
 
+        repo.write("file.txt", "first\n")
+        repo.jj({ "describe", "-m", "the very first change" })
+
+        local adapter = repo.adapter()
+        local commit_hash = run({ "jj", "show", "-T", "commit_id", "@", "--no-patch" }, repo.dir)
+        -- Initial commit's parent is the synthetic root; its commit_id IS
+        -- `NULL_TREE_SHA`. The description should still surface.
+        local left = adapter.Rev(RevType.COMMIT, adapter.Rev.NULL_TREE_SHA)
+        local right = adapter.Rev(RevType.COMMIT, commit_hash)
+
+        eq("the very first change", adapter:rev_to_panel_name(nil, left, right))
+      end)
+
+      it("routes through `parse_revs` for the working-copy `:DiffviewOpen` case", function()
+        if not jj_available() then
+          pending("jj not installed")
+          return
+        end
+
+        -- End-to-end coverage of the primary use case: `parse_revs(nil, {})`
+        -- returns a LOCAL right whose `object_name()` is `"UNKNOWN"`, so
+        -- `single_change_subject` must reach jj via the `@` fallback.
         repo.write("file.txt", "first\n")
         repo.jj({ "describe", "-m", "initial" })
         repo.jj({ "new" })
         repo.write("file.txt", "second\n")
-        repo.jj({ "describe", "-m", "current change" })
+        repo.jj({ "describe", "-m", "working copy change" })
 
         local adapter = repo.adapter()
         local left, right = adapter:parse_revs(nil, {})
 
-        eq(RevType.COMMIT, left.type)
-        eq(RevType.LOCAL, right.type)
-        eq("current change", adapter:rev_to_panel_name(nil, left, right))
+        eq("working copy change", adapter:rev_to_panel_name(nil, left, right))
       end)
 
-      it("falls back for null-tree ranges", function()
+      it("falls back to `rev_arg` when left is not a parent of right", function()
         if not jj_available() then
           pending("jj not installed")
           return
         end
 
+        -- Three-commit chain so grandparent is a real (non-null-tree) commit.
         repo.write("file.txt", "first\n")
-        repo.jj({ "describe", "-m", "initial" })
+        repo.jj({ "describe", "-m", "first" })
+        repo.jj({ "new" })
+        repo.write("file.txt", "second\n")
+        repo.jj({ "describe", "-m", "second" })
+        repo.jj({ "new" })
+        repo.write("file.txt", "third\n")
+        repo.jj({ "describe", "-m", "third" })
 
         local adapter = repo.adapter()
+        local grandparent_hash =
+          run({ "jj", "show", "-T", "commit_id", "@--", "--no-patch" }, repo.dir)
         local commit_hash = run({ "jj", "show", "-T", "commit_id", "@", "--no-patch" }, repo.dir)
-        local left = adapter.Rev.new_null_tree()
+        local non_parent = adapter.Rev(RevType.COMMIT, grandparent_hash)
         local right = adapter.Rev(RevType.COMMIT, commit_hash)
 
-        eq("root range", adapter:rev_to_panel_name("root range", left, right))
+        eq("main..@", adapter:rev_to_panel_name("main..@", non_parent, right))
       end)
     end)
 

lua/diffview/tests/functional/set_revs_spec.lua

@@ -62,6 +62,14 @@ describe("DiffView:set_revs", function()
       rev_to_pretty_string = function(_, left, right)
         return (left.commit or "?") .. ".." .. (right.commit or "?")
       end,
+      -- Distinctive sentinel that encodes all three inputs so tests catch
+      -- both a regression that copies raw `rev_arg` into the panel without
+      -- consulting the adapter, and one that forwards the wrong `left`/
+      -- `right` (e.g., pre-update values).
+      rev_to_panel_name = function(self, rev_arg, left, right)
+        return rev_arg
+          and ("PANEL[" .. rev_arg .. "|" .. self:rev_to_pretty_string(left, right) .. "]")
+      end,
       instanceof = function()
         return false
       end,
@@ -131,27 +139,9 @@ describe("DiffView:set_revs", function()
 
       view:set_revs("ccc333..ddd444")
 
-      eq("ccc333..ddd444", view.panel.rev_pretty_name)
-    end)
-
-    it("uses adapter panel labels when available", function()
-      local old_left = make_rev("aaa111")
-      local old_right = make_rev("bbb222")
-      local new_left = make_rev("ccc333")
-      local new_right = make_rev("ddd444")
-
-      local adapter = make_adapter({
-        ["ccc333..ddd444"] = { new_left, new_right },
-      })
-      adapter.rev_to_panel_name = function(_, rev_arg, left, right)
-        return table.concat({ "label", rev_arg, left.commit, right.commit }, ":")
-      end
-
-      local view = make_view(adapter, old_left, old_right, "aaa111..bbb222")
-
-      view:set_revs("ccc333..ddd444")
-
-      eq("label:ccc333..ddd444:ccc333:ddd444", view.panel.rev_pretty_name)
+      -- Prove the adapter method is consulted with the NEW `left`/`right`
+      -- (post-update); the mock's sentinel encodes all three inputs.
+      eq("PANEL[ccc333..ddd444|ccc333..ddd444]", view.panel.rev_pretty_name)
     end)
 
     it("does nothing when parse_revs fails", function()

lua/diffview/vcs/adapter.lua

@@ -654,8 +654,8 @@ function VCSAdapter:rev_to_pretty_string(left, right)
 end
 
 ---Convert revs to the display-only label shown in the file panel.
----Unlike rev_to_pretty_string the result need not be parseable as a rev.
----The default prefers the user's rev_arg; adapters may substitute a
+---Unlike `rev_to_pretty_string`, the result need not be parseable as a rev.
+---The default prefers the user's `rev_arg`; adapters may substitute a
 ---friendlier label.
 ---@param rev_arg string?
 ---@param left Rev

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

@@ -576,33 +576,32 @@ end
 ---@param right Rev
 ---@return string|nil
 function JjAdapter:single_change_subject(left, right)
-  if not (left and left.commit and right) then
+  -- Preconditions: `left` is a resolved commit, `right` is either a resolved
+  -- commit or LOCAL (which spells `@` on the CLI); reject the null-tree
+  -- sentinel because `jj show` on it emits a template error.
+  if not left.commit or right.commit == JjRev.NULL_TREE_SHA then
     return
   end
-
-  if left.commit == JjRev.NULL_TREE_SHA then
-    return
-  end
-
-  local right_rev
-  if right.type == RevType.LOCAL then
-    right_rev = "@"
-  elseif right.commit and right.commit ~= JjRev.NULL_TREE_SHA then
-    right_rev = right.commit
-  else
+  local right_arg = right.commit or (right.type == RevType.LOCAL and "@")
+  if not right_arg then
     return
   end
 
+  -- Emit `parent_hash\ndescription`. `\n` is officially supported in jj
+  -- template strings, and `description.first_line()` guarantees no embedded
+  -- newlines, so the output splits cleanly across two lines. `self:args()`
+  -- carries any user-configured global flags (e.g., `--repository`) ahead
+  -- of the subcommand, matching how `get_show_args`/`get_log_args` build
+  -- their invocations.
   local out, code = self:exec_sync(
     utils.vec_join(
       self:args(),
       "--ignore-working-copy",
-      "log",
-      "--no-graph",
-      "-r",
-      right_rev,
+      "show",
+      "--no-patch",
       "-T",
-      [[parents.first().commit_id() ++ "\x1f" ++ description.first_line() ++ "\n"]]
+      [[parents.first().commit_id() ++ "\n" ++ description.first_line()]],
+      right_arg
     ),
     {
       cwd = self.ctx.toplevel,
@@ -612,19 +611,30 @@ function JjAdapter:single_change_subject(left, right)
     }
   )
 
-  if code ~= 0 or not out[1] then
+  -- Explicit `not out` catches the bootstrap-fail path where `exec_sync`
+  -- returns bare nils; we don't want to rely on `code ~= 0`'s short-circuit
+  -- to shield the following `out[1]` index.
+  if code ~= 0 or not out or not out[1] then
     return
   end
 
-  local parent, subject = vim.trim(out[1]):match("^([^\31]*)\31(.*)$")
+  local parent = vim.trim(out[1])
   if parent ~= left.commit then
     return
   end
 
-  subject = vim.trim(subject or "")
-  return subject ~= "" and subject or nil
+  local subject = vim.trim(out[2] or "")
+  if subject == "" then
+    return
+  end
+
+  return subject
 end
 
+---Convert revs to the display-only label shown in the file panel.
+---Unlike `rev_to_pretty_string`, the result need not be parseable as a rev.
+---For a single-change range, use the change description; otherwise fall
+---back to the caller's `rev_arg` or the default pretty string.
 ---@param rev_arg string?
 ---@param left Rev
 ---@param right Rev