[vim/vim] Weired behaviour when using undercurl on Windows (Issue #15947)

Matthias

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Oct 27, 2024, 7:39:36 AM10/27/24

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Steps to reproduce

I was experimenting with the undercurl functionality on Windows using Windows Terminal that would support it since recently.

I enabled it by adding the following lines in my vimrc:

  let &t_Us = "\e[4:2m"
  let &t_ds = "\e[4:4m"
  let &t_Ds = "\e[4:5m"
  let &t_Cs = "\e[4:3m"
  let &t_Ce = "\e[4:0m"
  let &t_8u = "\e[58:2::%lu:%lu:%lum"

This works perfectly fine in my vim 8.2 running in WSL. But in vim 9.1 on Windows this leads to unexpected side effect. While it does indeed undercurl spelling mistakes, as soon as something is undercurled the whole terminal starts flickering wildly in insert mode and sometimes the colors just invert.

I first assumed that it may be a Windows Terminal bug, but as this behaviour does not appear in WSL, also using Windows Terminal, it may very well also be an issue inside vim.

https://github.com/user-attachments/assets/de0bfe76-2b0a-4516-b5ad-fe4db19aadcc

Expected behaviour

I would expect using undercurl not to have any weird side effects.

Version of Vim

9.1.813 on Windows / 8.2 on WSL (Ubuntu 22.04)

Environment

Windows Terminal 1.21

Logs and stack traces

No response

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Christian Brabandt

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Oct 27, 2024, 3:43:17 PM10/27/24

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can you reproduce this on other terminals?

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Matthias

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Oct 28, 2024, 2:32:01 AM10/28/24

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In vim 9 on Xubuntu it works fine in the xfce-terminal.

Regarding Windows I don't know if any other terminal there actually supports undercurl and so on.

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Matthias

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Oct 28, 2024, 3:42:37 PM10/28/24

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I installed WezTerm and Hyper and both produce the similar issues (though both do not support the displaying of undercurl).

I tried the following minimal special vimrc:

set nocompatible

set termguicolors


let &t_Us = "\e[4:2m"
let &t_ds = "\e[4:4m"
let &t_Ds = "\e[4:5m"
let &t_Cs = "\e[4:3m"
let &t_Ce = "\e[4:0m"
let &t_8u = "\e[58:2::%lu:%lu:%lum"

highlight SpellBad cterm=undercurl
highlight SpellCap cterm=undercurl

Then I started vim with vim --clean -u special-vimrc to be sure my other config is not to blame. Then type some nonsense text and finally activate set spell. Then continue typing in insert mode and witness things becoming weird.

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Dougaak

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Apr 27, 2025, 10:25:47 PM4/27/25

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Dougaak left a comment (vim/vim#15947)

Also having this issue, I use vim in Powershell + Windows Terminal natively. Is there any progress on this issue? Would really love to be able to use undercurl.

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Taylor Buchanan

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6:03 PM (4 hours ago) 6:03 PM

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gtbuchanan left a comment (vim/vim#15947)

Windows Terminal and WezTerm Nightly both display the underlines correctly when running the following command in PowerShell (pwsh):

Write-Host "`e[58:2::255:0:0m`e[4:1msingle`e[4:2mdouble`e[4:3mcurly`e[4:4mdotted`e[4:5mdashed`e[0m"

image.png (view on web)

However, Vim 9.1 still exhibits strange behavior. I haven't tried an isolated .vimrc, but in my main .vimrc it manifests as the cursor jumping everywhere and my text being mostly jumbled.

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Taylor Buchanan

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9:19 PM (1 hour ago) 9:19 PM

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I have the same issue on Windows with WezTerm Nightly. Running the following command in PowerShell (pwsh in WezTerm and Windows Terminal) outputs the appropriate underlines:

```

Write-Host "`e[58:2::255:0:0m`e[4:1msingle`e[4:2mdouble`e[4:3mcurly`e[4:4mdotted`e[4:5mdashed`e[0m"

```

However, Vim 9.1 completely freaks out

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