fix(picker): format fzf preview entries to list width

lua/opencode/ui/base_picker.lua

@@ -515,17 +515,6 @@ local function fzf_ui(opts)
   local fzf_config = create_fzf_config()
   fzf_config.actions = actions_config
 
-  -- When a preview pane is active, fzf-lua splits the window (default
-  -- right:60% preview, left:40% list). Narrow opts.width so that the
-  -- format function produces entries sized for the list pane, not the
-  -- full window. The format closure reads opts.width on each call.
-  local has_preview = opts.preview and opts.preview ~= 'none' and opts.preview ~= false
-  if has_preview and opts.width then
-    local window_cols = opts.width + 8
-    -- list pane ≈ 40% of the window, minus a small border/padding allowance
-    opts.width = math.floor(window_cols * 0.4) - 4
-  end
-
   fzf_lua.fzf_exec(create_finder(), fzf_config)
 end
 
@@ -890,6 +879,14 @@ function M.pick(opts)
     end
   end
 
+  local has_preview = opts.preview and opts.preview ~= 'none' and opts.preview ~= false
+  if picker_type == 'fzf' and has_preview and format_width then
+    local window_cols = format_width + 8
+    -- Match fzf-lua's default right:60% preview split so item formatting
+    -- targets the visible list pane instead of the full window width.
+    format_width = math.floor(window_cols * 0.4) - 4
+  end
+
   local original_format_fn = opts.format_fn
   opts.format_fn = function(item)
     return original_format_fn(item, format_width)

tests/unit/base_picker_spec.lua

@@ -233,6 +233,7 @@ end)
 
 describe('opencode.ui.base_picker fzf-lua preview', function()
   local base_picker
+  local captured_fzf_finder
   local captured_fzf_opts
   local original_schedule
   local saved_modules
@@ -290,10 +291,12 @@ describe('opencode.ui.base_picker fzf-lua preview', function()
       end,
     }
 
+    captured_fzf_finder = nil
     captured_fzf_opts = nil
 
     package.loaded['fzf-lua'] = {
-      fzf_exec = function(_, opts)
+      fzf_exec = function(finder, opts)
+        captured_fzf_finder = finder
         captured_fzf_opts = opts
       end,
     }
@@ -386,4 +389,32 @@ describe('opencode.ui.base_picker fzf-lua preview', function()
 
     vim.api.nvim_buf_delete(next_preview_buf, { force = true })
   end)
+
+  it('formats entries to the visible list width when preview is active', function()
+    local observed_width
+
+    base_picker.pick({
+      title = 'Test',
+      items = { { name = 'session' } },
+      format_fn = function(item, width)
+        observed_width = width
+        return base_picker.create_picker_item({ { text = item.name } })
+      end,
+      actions = {},
+      callback = function() end,
+      preview = 'custom',
+      preview_fn = function() end,
+    })
+
+    local emitted_lines = {}
+    captured_fzf_finder(function(line)
+      if line then
+        table.insert(emitted_lines, line)
+      end
+    end)
+
+    assert.equal(31, observed_width)
+    assert.are.same({ '1\001session' }, emitted_lines)
+    assert.equal(88, captured_fzf_opts.winopts.width)
+  end)
 end)