fix: preserve user's existing fillchars in output window

This should fix #415

Author: sudo-tee

lua/opencode/ui/output_window.lua

@@ -11,6 +11,13 @@ M._last_visible_bottom_by_win = {}
 M._was_at_bottom_by_win = {}
 M._prev_line_count_by_win = {}
 
+local OUTPUT_FOLD_FILLCHARS = {
+  fold = '-',
+  foldclose = '+',
+  foldopen = '-',
+  foldsep = '│',
+}
+
 local function build_fold_state(folds)
   local fold_state = {
     ranges = {},
@@ -258,7 +265,15 @@ function M.setup(windows)
   window_options.set_window_option('foldenable', true, windows.output_win)
   window_options.set_window_option('foldlevel', 0, windows.output_win)
   window_options.set_window_option('foldcolumn', '1', windows.output_win)
-  window_options.set_window_option('fillchars', 'fold:-,foldopen:-,foldclose:+,foldsep:│', windows.output_win)
+  window_options.set_window_option(
+    'fillchars',
+    vim.api.nvim_get_option_value('fillchars', { win = windows.output_win }),
+    windows.output_win,
+    { save_original = true }
+  )
+  vim.api.nvim_win_call(windows.output_win, function()
+    vim.opt_local.fillchars:append(OUTPUT_FOLD_FILLCHARS)
+  end)
   window_options.set_window_option('foldtext', 'v:lua.opencode_fold_text()', windows.output_win)
 
   if config.ui.position ~= 'current' then

tests/unit/output_window_spec.lua

@@ -4,6 +4,19 @@ local output_window = require('opencode.ui.output_window')
 local flush = require('opencode.ui.renderer.flush')
 local stub = require('luassert.stub')
 
+---@param value string
+---@return table<string, string>
+local function parse_fillchars(value)
+  local result = {}
+  for _, part in ipairs(vim.split(value, ',', { plain = true, trimempty = true })) do
+    local key, item = part:match('^([^:]+):(.*)$')
+    if key and item then
+      result[key] = item
+    end
+  end
+  return result
+end
+
 describe('output_window.create_buf', function()
   local original_config
 
@@ -154,12 +167,21 @@ describe('output_window.setup', function()
     assert.equals(0, foldlevel)
   end)
 
-  it('sets fold fillchars to avoid numeric fold column markers', function()
+  it('preserves existing fillchars while setting fold fillchars', function()
+    vim.api.nvim_set_option_value('fillchars', 'eob:~,lastline:@', { win = win, scope = 'local' })
+
     output_window.setup({ output_buf = buf, output_win = win })
 
     local fillchars = vim.api.nvim_get_option_value('fillchars', { win = win })
 
-    assert.equals('fold:-,foldopen:-,foldclose:+,foldsep:│', fillchars)
+    assert.same({
+      eob = '~',
+      lastline = '@',
+      fold = '-',
+      foldopen = '-',
+      foldclose = '+',
+      foldsep = '│',
+    }, parse_fillchars(fillchars))
   end)
 
   it('applies closed folds immediately when fold ranges change', function()