feat(float): open float relative to mouse #21531

Problem:
No easy way to position a LSP hover window relative to mouse.

Solution:
Introduce another option to the `relative` key in `nvim_open_win()`.

With this PR it should be possible to override the handler and do something
similar to this https://github.com/neovim/neovim/pull/19481#issuecomment-1193248674
to have hover information displayed from the mouse.

Test case:

    ```lua
    local util = require('vim.lsp.util')

    local function make_position_param(window, offset_encoding)
        window = window or 0
        local buf = vim.api.nvim_win_get_buf(window)
        local row, col

        local mouse = vim.fn.getmousepos()
        row = mouse.line
        col = mouse.column

        offset_encoding = offset_encoding or util._get_offset_encoding(buf)
        row = row - 1
        local line = vim.api.nvim_buf_get_lines(buf, row, row + 1, true)[1]
        if not line then
            return { line = 0, character = 0 }
        end
        if #line < col then
            return { line = 0, character = 0 }
        end

        col = util._str_utfindex_enc(line, col, offset_encoding)

        return { line = row, character = col }
    end

    local make_params = function(window, offset_encoding)
        window = window or 0
        local buf = vim.api.nvim_win_get_buf(window)
        offset_encoding = offset_encoding or util._get_offset_encoding(buf)
        return {
            textDocument = util.make_text_document_params(buf),
            position = make_position_param(window, offset_encoding),
        }
    end

    local hover_timer = nil
    vim.o.mousemoveevent = true

    vim.keymap.set({ '', 'i' }, '<MouseMove>', function()
        if hover_timer then
            hover_timer:close()
        end
        hover_timer = vim.defer_fn(function()
            hover_timer = nil
            local params = make_params()
            vim.lsp.buf_request(
                0,
                'textDocument/hover',
                params,
                vim.lsp.with(vim.lsp.handlers.hover, {
                    silent = true,
                    focusable = false,
                    relative = 'mouse',
                })
            )
        end, 500)
        return '<MouseMove>'
    end, { expr = true })
    ```

3 files changed, 6 insertions, 0 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index c827da4554..d74657dc8e 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -2995,6 +2995,7 @@ nvim_open_win({buffer}, {enter}, {*config})                  *nvim_open_win()*
                     • "win" Window given by the `win` field, or current
                       window.
                     • "cursor" Cursor position in current window.
+                    • "mouse" Mouse position
 
                   • win: |window-ID| for relative="win".
                   • anchor: Decides which corner of the float to place at
diff --git a/runtime/doc/lsp.txt b/runtime/doc/lsp.txt
index cf82cbf204..aff0f9a793 100644
--- a/runtime/doc/lsp.txt
+++ b/runtime/doc/lsp.txt
@@ -1612,6 +1612,7 @@ make_floating_popup_options({width}, {height}, {opts})
                   • border (string or table) override `border`
                   • focusable (string or table) override `focusable`
                   • zindex (string or table) override `zindex`, defaults to 50
+                  • relative ("mouse"|"cursor") defaults to "cursor"
 
     Return: ~
         (table) Options
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 88bad1b7df..7dceaa3318 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -50,6 +50,10 @@ NEW FEATURES                                                    *news-features*
 
 The following new APIs or features were added.
 
+• |nvim_open_win()| now accepts a relative `mouse` option to open a floating win
+  relative to the mouse. Note that the mouse doesn't update frequently without
+  setting `vim.o.mousemoveevent = true`
+
 • EditorConfig support is now builtin. This is enabled by default and happens
   automatically. To disable it, users should add >lua