Invalid window id when the context window closes mid-render

Description

Render.open validates window_context.context_winid at render.lua:518, then runs set_lines(), nvim_buf_clear_namespace(), highlight_contexts(), copy_extmarks() and highlight_bottom() before handing that same handle to horizontal_scroll_contexts() at render.lua:532, which calls nvim_win_call on it.

The window can close in between. set_lines() writes vim.bo[bufnr].modifiable, which fires OptionSet; any handler that closes a window during that event can take the context float with it, since it is relative='win'. The check at 518 is real but too early to cover the use at 532.

The guard at treesitter-context.lua:104 does not help either: it validates winid, before context.get() and the whole render.

At the point of failure winid is still valid and context_winid is not, so the id in the message is the context float.

Neovim version

NVIM v0.12.4, LuaJIT 2.1.1774638290

Expected behavior

A context window closing mid-render aborts that render quietly, the way render.lua:518 already handles the same window being gone.

Actual behavior

vim.schedule callback: .../render.lua:382: Invalid window id: 1003
stack traceback:
	[C]: in function 'nvim_win_call'
	.../render.lua:382: in function 'horizontal_scroll_contexts'
	.../render.lua:532: in function 'open'
	.../treesitter-context.lua:120: in function 'f'
	.../treesitter-context.lua:60: in function <.../treesitter-context.lua:40>

Reproduced on master f3061339.

Minimal config

local plugins = {
  ts         = 'https://github.com/nvim-treesitter/nvim-treesitter',
  ts_context = 'https://github.com/nvim-treesitter/nvim-treesitter-context',
}

for name, url in pairs(plugins) do
  local install_path = '/tmp/nvim/site/'..name
  if vim.fn.isdirectory(install_path) == 0 then
    vim.fn.system { 'git', 'clone', '--depth=1', url, install_path }
  end
  vim.o.runtimepath = install_path..','..vim.o.runtimepath
end

require('treesitter-context').setup({})

-- A file deep enough to have a context, written here so the repro is one file.
local body = {}
for i = 1, 80 do
  body[#body + 1] = ('      local x%d = %d'):format(i, i)
end
vim.fn.writefile(
  vim.list_extend({
    'local function outer()',
    '  local function middle()',
    '    local function inner()',
  }, vim.list_extend(body, { '    end', '  end', 'end' })),
  '/tmp/nvim/sample.lua')

-- Stand-in for whatever closed the window in the wild: fires once, during the
-- render, from the 'modifiable' toggle in set_lines().
local fired = false
vim.api.nvim_create_autocmd('OptionSet', {
  pattern = 'modifiable',
  callback = function()
    if fired or #vim.api.nvim_list_wins() < 2 then return end
    fired = true
    pcall(vim.api.nvim_win_close, vim.api.nvim_get_current_win(), true)
  end,
})

Steps to reproduce

1. nvim --clean -u minimal.lua /tmp/nvim/sample.lua
2. :vsplit
3. 60G
4. :messages

The OptionSet autocmd is a stand-in that makes the timing deterministic. This started from an organic occurrence in a large config, which I could not isolate to a specific plugin; the autocmd reproduces the same traceback on demand.

Suggested fix

local function horizontal_scroll_contexts(winid, context_winid)
  if not api.nvim_win_is_valid(winid) or not api.nvim_win_is_valid(context_winid) then
    return
  end

Verified against f3061339: the repro is clean with this applied, and contexts still render normally. Guarding only winid is not enough, the error moves to the next line.

Start in render.lua at horizontal_scroll_contexts() and its call from Render.open around lines 518 and 532. Run the minimal configuration from the issue to reproduce the mid-render window close. Done means the context window closing during rendering aborts quietly and the repro produces no invalid-window error while normal contexts still render.