Hmm, when I try to use default keymaps nothing happens.

I'm having similar behavior, it seems to me that it toggles the previous selection in visual/visual block mode, ignoring the visual line mode selection.

ETA: I don't know why I thought this, but it looks like the plugin expects you to leave visual mode before calling require("Comment.api").toggle.linewise(vim.fn.visualmode()) as it uses vim.api.nvim_buf_get_mark(0, '<'), which doesn't get the current selection, but the previous one. Also, I don't believe it works correctly with visual block mode, as it doesn't recognize ^V as a visual mode.

Surprisingly, I just had a similar dive into how to get visual region.

Yeah, this plugin is also making the same mistake, so either nvim changed it recently or just common misconception.

Need to either change to . and v marks or modify function to exit visual mode before calling toggle.linewise():

{
    "<C-_>",
    function()
        vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes("<Esc>", true, true, true), "n", true)
        require("Comment.api").toggle.linewise(vim.fn.visualmode())
    end,
    mode = "v",
    desc = "Comment selection",
},

I just came to the same conclusion. I don't think this plugin is making a mistake so much as just expecting you to exit visual mode first. Trying to adjust the plugin to use . and v positions.

@Rizhiy for some reason that's not working here.

I have the following in my keybinds.lua:

vim.keymap.set('v', "«", function()
  vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes("<Esc>", true, true, true), "n", true)
  require('Comment.api').toggle.linewise(vim.fn.visualmode())
end)

(for some reason option-\ ends up as being « on my system, don't ask).

It still comments strange parts of the file in visual mode.