Last line is sometimes not rendered

[simonmandlik]

I have noticed that the column on the last line is sometimes rendered and sometimes it isn't.

MWE:

local root = vim.fn.fnamemodify("./.repro", ":p")
for _, name in ipairs({ "config", "data", "state", "cache" }) do
  vim.env[("XDG_%s_HOME"):format(name:upper())] = root .. "/" .. name
end
local lazypath = root .. "/plugins/lazy.nvim"
if not vim.loop.fs_stat(lazypath) then
  vim.fn.system({ "git", "clone", "--filter=blob:none", "https://github.com/folke/lazy.nvim.git", lazypath, })
end
vim.opt.runtimepath:prepend(lazypath)

local plugins = {
    spec = {
        {
            "lukas-reineke/virt-column.nvim",
            opts = {
                virtcolumn = "+1,+2,+3",
            },
        },
    }
}

require("lazy").setup(plugins, {
    root = root .. "/plugins",
})

vim.opt.textwidth = 10

Run nvim -u repro.lua.

1. In insert mode, write return {\n}. This leads to this:

2. In empty file again, write return {\n\n<BS>} in insert mode. This gives me:

Two buffers are identical, but virtual column is different

[IronGeek]

Hi, a user who's also having the same issue here.

Anyway, this past weekend I decided to run the trusted print debugger with this plugin, and I think I may have found the culprit. It's the infamous off by one error.

Bellow is a screenshot of Neovim and virt-column.nvim loaded with the following options:

{  
  char = "...", -- hardcoded to `"i=?, [top,bottom]=[?,?]"` in `lua/virt-column/init.lua`
  virtcolumn = "30"
}

lua/virt-column/init.lua#85:

- utils.tbl_get_index(char, column_index),
+ string.format("i=>%d, [top,bottom]=>[%d,%d]", i, topline, botline_guess),

Uncleared last line

From the above picture, notice that extmark for the last line is never updated, or to be precise it is sometimes rendered, but never cleared. That is because of how the nvim_buf_clear_namespace() function is called on lua/virt-column/init.lua#62:

virt-column.nvim/lua/virt-column/init.lua

Line 62 in b87e3e0

pcall(vim.api.nvim_buf_clear_namespace, bufnr, M.namespace, topline, botline_guess)

According to Neovim help doc, the lines argument in the nvim_buf_clear_namespace({buffer}, {ns_id}, {line_start}, {line_end}) are 0-indexed, and the line_end argument the specify the end of range of lines to clear is exclusive.

So, if we're trying to clear 5 lines from the top: topline = 0 and botline_guess = 4 (remember 0-indexed), then calling:

pcall(
    vim.api.nvim_buf_clear_namespace, 
    bufnr, 
    M.namespace, 
    0, -- topline
    4, -- botline_guess
)

would actually clear line[0], line[1], line[2], and line[3]... leaving line[4] untouched because nvim_buf_clear_namespace() use end-exclusive range.

By changing the argument to botline_guess + 1 on lua/virt-column/init.lua#62:

- pcall(vim.api.nvim_buf_clear_namespace, bufnr, M.namespace, topline, botline_guess)
+ pcall(vim.api.nvim_buf_clear_namespace, bufnr, M.namespace, topline, botline_guess + 1)

The whole 5 lines would now be correctly cleared.

Unrendered last line

The last line is now correctly cleared, but there's still remaining issue, the extrmark for the last line is never rendered.
If we look at the extmark from the screenshots we can see that i is always started at 1 eventough the topline variable always start at 0. That's because on every iteration we're actually rendering the extmark to the previous line instead of rendering the extmark to the current line:

virt-column.nvim/lua/virt-column/init.lua

Line 82 in b87e3e0

pcall(vim.api.nvim_buf_set_extmark, bufnr, M.namespace, i - 1, 0, {

According to Neovim help doc, the nvim_buf_set_extmark({buffer}, {ns_id}, {line}, {col}, {opts}) function use extmarks indexing (also a 0-based indices), so theres shouldn't be any need to map (add offset) between the topline variable (or the i variable) to the line variable in nvim_buf_set_extmark().

By removing the offset added to i on lua/virt-column/init.lua#82, we could see the extmark is correctly lined, updated, and cleared with every render.

lua/virt-column/init.lua#L82:

- pcall(vim.api.nvim_buf_set_extmark, bufnr, M.namespace, i - 1, 0, {
+ pcall(vim.api.nvim_buf_set_extmark, bufnr, M.namespace, i, 0, {

Additional ones

We could see the issue is gone now, but there's still one more potential problem we need to take care.
To make the fix complete, we also need to make changes to the following:

lua/virt-column/init.lua#78:

virt-column.nvim/lua/virt-column/init.lua

Line 78 in b87e3e0

return vim.fn.virtcol { i, "$" } - 1

- return vim.fn.virtcol { i, "$" } - 1
+ return vim.fn.virtcol { i + 1, "$" } - 1

lua/virt-column/init.lua#98:

virt-column.nvim/lua/virt-column/init.lua

Line 98 in b87e3e0

return vim.fn.foldclosedend(i)

- return vim.fn.foldclosedend(i)
+ return vim.fn.foldclosedend(i + 1)

and, lua/virt-column/init.lua#101:

virt-column.nvim/lua/virt-column/init.lua

Line 101 in b87e3e0

i = fold_end

- i = fold_end
+ i = fold_end - 1

AFAIK, the vim.fn.virtcol() and vim.fn.foldclosedend() uses 1-based indexing, and since the i variable is 0-indexed, so we do need to actually add an offset here.

---

I've already submitted a PR for this. I hope my analysis is correct on this one. Thank You.