fix(ui): correct action & task line offsets

Adjust indexing used when anchoring actions and task ranges so actions
and extmarks align with their rendered lines:

- formatter.add_action: use (line or output:get_line_count()) - 1
- formatter/tools/task: set display_line = start_line and range = { from = start_line + 1, to = end_line + 1 }
- renderer/buffer: pass line_start to add_actions instead of line_start + 1
- renderer: avoid inserting synthetic revert message into state.messages; notify via events only

Add tests:
- unit tests for formatter anchoring and assistant-mode fallback
- replay test asserting a single synthetic revert message is produced

Fixes off-by-one rendering/anchor issues and adds tests to prevent regressions.

Author: sudo-tee

lua/opencode/ui/formatter.lua

@@ -101,7 +101,7 @@ end
 
 local function add_action(output, text, action_type, args, key, line)
   -- actions use api-indexing (e.g. 0 indexed)
-  line = (line or output:get_line_count()) - 2
+  line = (line or output:get_line_count()) - 1
   output:add_action({
     text = text,
     type = action_type,

lua/opencode/ui/formatter/tools/task.lua

@@ -79,8 +79,8 @@ function M.format(output, part, get_child_parts)
     type = 'select_child_session',
     args = {},
     key = 'S',
-    display_line = start_line - 1,
-    range = { from = start_line, to = end_line },
+    display_line = start_line,
+    range = { from = start_line + 1, to = end_line + 1 },
   })
 end
 

lua/opencode/ui/renderer.lua

@@ -156,7 +156,6 @@ function M._render_full_session_data(session_data)
       },
     }
 
-    table.insert(state.messages, revert_message)
     events.on_message_updated(revert_message)
     events.on_part_updated({ part = revert_message.parts[1] })
   end

lua/opencode/ui/renderer/buffer.lua

@@ -276,7 +276,7 @@ end
 local function apply_part_actions(part_id, formatted_data, line_start)
   if has_actions(formatted_data.actions) then
     ctx.render_state:clear_actions(part_id)
-    ctx.render_state:add_actions(part_id, vim.deepcopy(formatted_data.actions), line_start + 1)
+    ctx.render_state:add_actions(part_id, vim.deepcopy(formatted_data.actions), line_start)
   else
     ctx.render_state:clear_actions(part_id)
   end

tests/replay/renderer_spec.lua

@@ -195,6 +195,36 @@ describe('renderer unit tests', function()
     render_stub:revert()
   end)
 
+  it('inserts a single synthetic revert message during full session render', function()
+    local renderer = require('opencode.ui.renderer')
+
+    helpers.replay_setup()
+
+    state.session.set_active({
+      id = 'ses_123',
+      title = 'Session',
+      time = { created = 1, updated = 1 },
+      revert = { messageID = 'msg_1', snapshot = 'a', diff = '' },
+    })
+
+    renderer._render_full_session_data({
+      {
+        info = {
+          id = 'msg_1',
+          role = 'assistant',
+          sessionID = 'ses_123',
+        },
+        parts = {},
+      },
+    })
+
+    local revert_messages = vim.tbl_filter(function(message)
+      return message.info and message.info.id == '__opencode_revert_message__'
+    end, state.messages or {})
+
+    assert.are.equal(1, #revert_messages)
+  end)
+
   it('ignores session.updated for non-active session IDs', function()
     local renderer = require('opencode.ui.renderer')
 

tests/unit/formatter_spec.lua

@@ -2,6 +2,7 @@ local assert = require('luassert')
 local config = require('opencode.config')
 local formatter = require('opencode.ui.formatter')
 local Output = require('opencode.ui.output')
+local state = require('opencode.state')
 
 describe('formatter', function()
   before_each(function()
@@ -238,4 +239,125 @@ describe('formatter', function()
     assert.are.equal('**  grep** `*.lua eventignore` 1s', output.lines[1])
     assert.are.equal('Found `3` matches', output.lines[2])
   end)
+
+  it('anchors snapshot actions to the snapshot and restore lines', function()
+    local snapshot = require('opencode.snapshot')
+    local original_get_restore_points_by_parent = snapshot.get_restore_points_by_parent
+
+    snapshot.get_restore_points_by_parent = function(hash)
+      if hash == 'abcdef123456' then
+        return {
+          {
+            id = 'restore123456',
+            created_at = 1,
+          },
+        }
+      end
+      return {}
+    end
+
+    local message = {
+      info = {
+        id = 'msg_1',
+        role = 'assistant',
+        sessionID = 'ses_1',
+      },
+      parts = {},
+    }
+
+    local part = {
+      id = 'prt_patch_1',
+      type = 'patch',
+      hash = 'abcdef123456',
+      messageID = 'msg_1',
+      sessionID = 'ses_1',
+    }
+
+    local output = formatter.format_part(part, message, true)
+
+    snapshot.get_restore_points_by_parent = original_get_restore_points_by_parent
+
+    assert.are.same({ 0, 0, 0, 1, 1 }, vim.tbl_map(function(action)
+      return action.display_line
+    end, output.actions))
+  end)
+
+  it('falls back to current mode for assistant messages without a stamped mode', function()
+    state.model.set_mode('build')
+    local output = formatter.format_message_header({
+      info = {
+        id = 'msg_current',
+        role = 'assistant',
+        sessionID = 'ses_1',
+      },
+      parts = {},
+    })
+
+    assert.are.equal('BUILD', output.extmarks[1][1].virt_text[3][1])
+  end)
+
+  it('anchors task child-session action to the rendered task block', function()
+    local message = {
+      info = {
+        id = 'msg_1',
+        role = 'assistant',
+        sessionID = 'ses_1',
+      },
+      parts = {},
+    }
+
+    local part = {
+      id = 'prt_task_1',
+      type = 'tool',
+      tool = 'task',
+      messageID = 'msg_1',
+      sessionID = 'ses_1',
+      state = {
+        status = 'completed',
+        input = {
+          description = 'review changes',
+          subagent_type = 'explore',
+        },
+        metadata = {
+          sessionId = 'ses_child',
+        },
+        time = {
+          start = 1,
+          ['end'] = 2,
+        },
+      },
+    }
+
+    local child_parts = {
+      {
+        id = 'prt_child_1',
+        type = 'tool',
+        tool = 'read',
+        messageID = 'msg_child_1',
+        sessionID = 'ses_child',
+        state = {
+          status = 'completed',
+          input = {
+            filePath = '/tmp/project',
+          },
+        },
+      },
+    }
+
+    local output = formatter.format_part(part, message, true, function(session_id)
+      if session_id == 'ses_child' then
+        return child_parts
+      end
+      return nil
+    end)
+
+    assert.are.same({
+      text = '[S]elect Child Session',
+      type = 'select_child_session',
+      args = {},
+      key = 'S',
+      display_line = 1,
+      range = { from = 2, to = 5 },
+    }, output.actions[1])
+  end)
 end)