Neovim undercurl still renders as a plain text-colored underline with Herdr 0.7.5 #2121

Is this a reproducible bug?

• I confirm this is a reproducible bug, not a feature request, idea, question, contribution proposal, or direction check.
• I reproduced this bug on the version and environment reported below using the exact steps provided.

Current behavior

Neovim 0.12.4 inside a default Herdr pane renders an undercurl highlight as a straight underline using the text color.

The same minimal configuration renders a colored curly underline when Neovim starts with TERM=xterm-kitty.

Expected behavior

The configured curly underline and special color render inside a default Herdr pane.

Reproduction

1. Save this as /tmp/undercurl.lua:

vim.o.termguicolors = true
vim.api.nvim_set_hl(0, "TestUndercurl", {
    undercurl = true,
    sp = "#6897BB",
})
vim.api.nvim_buf_set_lines(0, 0, -1, false, { "typoo" })
vim.api.nvim_buf_set_extmark(
    0,
    vim.api.nvim_create_namespace("undercurl-test"),
    0,
    0,
    { end_col = 5, hl_group = "TestUndercurl" }
)

2. Start Neovim inside Herdr:

nvim --clean -u /tmp/undercurl.lua

3. Observe a straight text-colored underline

4. Quit and start the same test with:

TERM=xterm-kitty nvim --clean -u /tmp/undercurl.lua

5. Observe a blue curly underline

Impact

Diagnostic underlines lose their configured shape and severity color inside Herdr.

Environment

• Herdr version: 0.7.5
• Update channel: stable
• Operating system: macOS 26.5.2
• Terminal: Kitty 0.48.1
• Neovim: 0.12.4
• Shell: zsh 5.9
• Relevant config: default Herdr pane environment with TERM=xterm-256color and COLORTERM=truecolor

Related:

• Herdr does not respond to terminal capability queries (XTGETTCAP, DSR, DA) #393
• Undercurl renders as plain underline in Herdr #895