fix(view): distinguish no-merge-base from probe errors in worktree overview

doc/diffview.txt

@@ -646,9 +646,9 @@ Column legend ~
                                            fallbacks; see `base` in
                                            |diffview-config-worktree_overview|).
                             `Nf`             Files changed in `base...HEAD`
-                                           per `git diff --shortstat`.
+                                           per `git diff --numstat`.
                             `+N` `-N`          Insertion / deletion counts
-                                           from the same shortstat.
+                                           from the same numstat.
                             `Ns` `Nu` `N?`       Dirty trio from
                                            `git status --porcelain`:
                                            staged (index-vs-HEAD),
@@ -665,6 +665,26 @@ Column legend ~
                                            every row stripped so the
                                            rows read as a tidy list.
 
+                        Numeric cells can collapse to one of two dim
+                        single-column markers in place of a digit:
+
+                            `×`              The probe failed, so the real
+                                           count is unknown. Treating it
+                                           as zero would misrepresent the
+                                           row as clean, so each affected
+                                           cell shows this instead.
+                            `·`              The question doesn't apply.
+                                           Currently only the diff cells
+                                           (`Nf`, `+N`, `-N`) use this,
+                                           on branches whose history is
+                                           unrelated to the resolved
+                                           base so `base...HEAD` has no
+                                           merge base and the diff is
+                                           undefined. Ahead / behind
+                                           still render since
+                                           `rev-list --left-right`
+                                           needs no merge base.
+
                                                 *:DiffviewLog*
 :DiffviewLog            Open the debug log.
 

lua/diffview/scene/views/worktree_overview/worktree_overview_view.lua

@@ -759,22 +759,22 @@ local function pad(ctx, n)
   end
 end
 
----Marker used when a probe is unavailable (e.g., no base resolved, or a
----sub-probe exited non-zero). Zero would silently misrepresent the row as
----clean, so each affected cell shows a dim dash instead of a digit.
-local UNKNOWN = "─"
-
----Resolve a counter cell's `(text, display_width, hl)` tuple. When the
----probe is unavailable the cell collapses to a dim `UNKNOWN` marker
----(display width 1); otherwise it uses the formatted value with the
----caller-supplied highlight.
----@param unknown boolean
+---Probe failed: real count unknown. Visually loud so a stale probe can't
+---pass for a clean row.
+local UNKNOWN = "×"
+
+---The question doesn't apply (e.g., diff cells on a row whose
+---`base...HEAD` has no merge base). Quieter than `UNKNOWN` since nothing
+---broke.
+local NA = "·"
+
+---@param marker? string # `UNKNOWN`, `NA`, or nil for "render the number".
 ---@param n integer
 ---@param hl string
 ---@return string text, integer width, string hl
-local function fmt_cell(unknown, n, hl)
-  if unknown then
-    return UNKNOWN, 1, HL.dim
+local function fmt_cell(marker, n, hl)
+  if marker then
+    return marker, 1, HL.dim
   end
   local s = tostring(n)
   return s, #s, hl
@@ -800,39 +800,42 @@ local function push_stats_cells(ctx, e, w)
   end
   local s = e.stats ---@cast s GitAdapter.WorktreeStats
 
-  -- Branch counters: nil base means the probes were skipped entirely, so
-  -- ahead/behind would silently read as 0; `errors` lists sub-probes that
-  -- exited non-zero. Either case renders the affected cells as unknown
-  -- so a stale probe doesn't masquerade as a clean row.
-  local branch_unknown = s.base == nil or vim.tbl_contains(s.errors, "rev-list")
-  local diff_unknown = s.base == nil or vim.tbl_contains(s.errors, "diff")
+  -- Nil base means probes were skipped; without a marker the row would
+  -- read as clean. Diff has the extra `no_merge_base` outcome.
+  local branch_marker = (s.base == nil or vim.tbl_contains(s.errors, "rev-list")) and UNKNOWN or nil
+  local diff_marker
+  if s.base == nil or vim.tbl_contains(s.errors, "diff") then
+    diff_marker = UNKNOWN
+  elseif s.no_merge_base then
+    diff_marker = NA
+  end
   local status_unknown = vim.tbl_contains(s.errors, "status")
 
   local text, cw, hl
-  text, cw, hl = fmt_cell(branch_unknown, s.ahead, HL.counter)
+  text, cw, hl = fmt_cell(branch_marker, s.ahead, HL.counter)
   pad(ctx, w.ahead - cw)
   push(ctx, "↑", HL.arrow)
   push(ctx, text, hl)
   push(ctx, " ")
 
-  text, cw, hl = fmt_cell(branch_unknown, s.behind, HL.counter)
+  text, cw, hl = fmt_cell(branch_marker, s.behind, HL.counter)
   pad(ctx, w.behind - cw)
   push(ctx, "↓", HL.arrow)
   push(ctx, text, hl)
   push(ctx, "  ")
 
-  text, cw, hl = fmt_cell(diff_unknown, s.changed_files, HL.counter)
+  text, cw, hl = fmt_cell(diff_marker, s.changed_files, HL.counter)
   pad(ctx, w.files - cw)
   push(ctx, text, hl)
   push(ctx, "f", HL.files_suffix)
   push(ctx, "  ")
 
-  text, cw, hl = fmt_cell(diff_unknown, s.insertions, HL.insertions)
+  text, cw, hl = fmt_cell(diff_marker, s.insertions, HL.insertions)
   pad(ctx, w.ins - cw)
   push(ctx, "+" .. text, hl)
   push(ctx, " ")
 
-  text, cw, hl = fmt_cell(diff_unknown, s.deletions, HL.deletions)
+  text, cw, hl = fmt_cell(diff_marker, s.deletions, HL.deletions)
   pad(ctx, w.dels - cw)
   push(ctx, "-" .. text, hl)
   push(ctx, "  ")
@@ -852,7 +855,8 @@ local function push_stats_cells(ctx, e, w)
     return
   end
   local function dirty(count, width, hl_known, suffix)
-    local t, w1, h = fmt_cell(status_unknown, count, count > 0 and hl_known or HL.dim)
+    local marker = status_unknown and UNKNOWN or nil
+    local t, w1, h = fmt_cell(marker, count, count > 0 and hl_known or HL.dim)
     pad(ctx, width - w1)
     push(ctx, t, h)
     push(ctx, suffix, h)

lua/diffview/tests/functional/worktree_overview_view_spec.lua

@@ -303,6 +303,7 @@ describe("diffview.scene.views.worktree_overview WorktreeOverviewView", function
             staged = 0,
             unstaged = 0,
             untracked = 0,
+            no_merge_base = false,
             errors = { "status" },
           },
         }
@@ -340,18 +341,18 @@ describe("diffview.scene.views.worktree_overview WorktreeOverviewView", function
         -- and the suffix letters `s`/`u`/`?` live only on this trio -- so
         -- a hit on each proves the `status_unknown` branch fired. Plain
         -- find keeps the `?` from being treated as a pattern.
-        assert.is_truthy(errored_line:find("─s", 1, true))
-        assert.is_truthy(errored_line:find("─u", 1, true))
-        assert.is_truthy(errored_line:find("─?", 1, true))
+        assert.is_truthy(errored_line:find("×s", 1, true))
+        assert.is_truthy(errored_line:find("×u", 1, true))
+        assert.is_truthy(errored_line:find("×?", 1, true))
 
         -- Fixture-independent check: anything the two rows share (nil
         -- base, probe errors on branch/diff) adds the same number of
-        -- dashes to both lines, so the delta isolates the trio's own
-        -- contribution. The errored row should carry exactly three
-        -- more dashes than the clean row.
-        local _, clean_dashes = clean_line:gsub("─", "")
-        local _, errored_dashes = errored_line:gsub("─", "")
-        assert.equals(3, errored_dashes - clean_dashes)
+        -- `UNKNOWN` markers to both lines, so the delta isolates the
+        -- trio's own contribution. The errored row should carry exactly
+        -- three more markers than the clean row.
+        local _, clean_markers = clean_line:gsub("×", "")
+        local _, errored_markers = errored_line:gsub("×", "")
+        assert.equals(3, errored_markers - clean_markers)
       end)
 
       test_utils.close_view(view)

lua/diffview/vcs/adapters/git/init.lua

@@ -522,7 +522,8 @@ end
 ---@field staged integer # Index-vs-HEAD changes from `git status --porcelain`.
 ---@field unstaged integer # Worktree-vs-index changes.
 ---@field untracked integer
----@field errors string[] # Tags of probes that returned non-zero: `"rev-list"`, `"diff"`, `"status"`. Non-empty means the counts are partial; callers that would act destructively on a seemingly clean worktree must treat that as unknown state. A `nil` base is not an error (branch probes are skipped and `errors` stays empty for them).
+---@field no_merge_base boolean # `base...HEAD` has no merge base (unrelated histories). The diff counts are undefined; `errors` stays clear of `"diff"` since the probe answered a well-defined question. Branch counters are unaffected.
+---@field errors string[] # Tags of probes that returned non-zero: `"rev-list"`, `"diff"`, `"status"`. Non-empty means the counts are partial; callers that would act destructively on a seemingly clean worktree must treat that as unknown state. A `nil` base is not an error (branch probes are skipped and `errors` stays empty); nor is `no_merge_base`.
 
 ---Build the (unstarted) `Job`s that `worktree_stats` would run and a parser
 ---that reads their outputs into a `WorktreeStats`. Returned jobs are not
@@ -575,6 +576,7 @@ function GitAdapter:build_stats_probe_jobs(toplevel, base)
       staged = 0,
       unstaged = 0,
       untracked = 0,
+      no_merge_base = false,
       errors = {},
     }
 
@@ -607,7 +609,13 @@ function GitAdapter:build_stats_probe_jobs(toplevel, base)
           end
         end
       else
-        table.insert(stats.errors, "diff")
+        -- `A...HEAD` without a merge base (unrelated histories) is a
+        -- well-defined n/a, not a probe error -- keep it off `errors`.
+        if table.concat(diff_job.stderr, "\n"):find("no merge base", 1, true) then
+          stats.no_merge_base = true
+        else
+          table.insert(stats.errors, "diff")
+        end
       end
     end