BUG: Search filter is case-sensitive for non-ASCII characters (Cyrillic)

[itrous]

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Neovim Version (nvim -v)

NVIM v0.11.5

Operating System / Version

Linux krl-arch 6.17.7-arch1-1

Describe the Bug

The fuzzy search/filter functionality in Neo-tree works case-insensitively for ASCII characters (English) but becomes case-sensitive for non-ASCII characters like Cyrillic.

Actual Behavior:

English: searching "test" matches "Test", "TEST", "TeSt" ✅ (works correctly)
Russian: searching "тест" does NOT match "Тест" or "ТЕСТ" ❌ (case-sensitive)

Screenshots, Traceback

No response

Steps to Reproduce

1. Open Neo-tree with a directory containing files with non-ASCII names (e.g., Russian/Cyrillic)
2. Use fuzzy finder (/) or filter
3. Try searching with lowercase letters

Expected Behavior

Search should be case-insensitive for all characters:
English: searching "test" should match "Test", "TEST", "TeSt" ✅
Russian: searching "тест" should match "Тест", "ТЕСТ", "ТеСт" ✅

Your Configuration

-- template from https://lazy.folke.io/developers#reprolua, feel free to replace if you have your own minimal init.lua
vim.env.LAZY_STDPATH = ".repro"
load(vim.fn.system("curl -s https://raw.githubusercontent.com/folke/lazy.nvim/main/bootstrap.lua"))()

require("lazy.minit").repro({
  spec = {
    {
      "nvim-neo-tree/neo-tree.nvim",
      branch = "v3.x", -- or "main"
      dependencies = {
        "nvim-lua/plenary.nvim",
        "nvim-tree/nvim-web-devicons", -- not strictly required, but recommended
        "MunifTanjim/nui.nvim",
        -- { "3rd/image.nvim", opts = {} }, -- Optional image support
      },
      lazy = false,
      ---@module "neo-tree"
      ---@type neotree.Config?
      opts = {
        -- fill any relevant options here
      },
    }
  },
})
vim.g.mapleader = " "
vim.keymap.set("n", "<leader>e", "<Cmd>Neotree<CR>")
-- do anything else you need to do to reproduce the issue

[pynappo]

thanks for the bug report, i can reproduce.

currently neo-tree wraps queries to automatically be *<term>* and sends them to fd/find to search by glob. assuming you're using fd on your system, i don't think fd does case-insensitive globs properly with cyrillic. this is what i'm seeing:

my test dir:

cyrillic/
├── ТЕСТ
├── Тест
└── тест

1 directory, 3 files

find works fine:

> find . -iname "*тест*"
./cyrillic/Тест
./cyrillic/ТЕСТ
./cyrillic/тест

fd does not:

> fd -i --glob "*тест*"
cyrillic/тест

i'll prob file a report but in the meantime, consider setting filesystem.find_command = "find"

edit: upon further investigation this seems to be because ripgrep's globset crate (which fd uses) does not respect POSIX locales and thus doesn't do case-insensitive matches for unicode?