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 })
    ```

9 files changed, 53 insertions, 5 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
 
diff --git a/runtime/lua/vim/lsp/handlers.lua b/runtime/lua/vim/lsp/handlers.lua
index 80df83732e..b383ca1c35 100644
--- a/runtime/lua/vim/lsp/handlers.lua
+++ b/runtime/lua/vim/lsp/handlers.lua
@@ -335,7 +335,9 @@ function M.hover(_, result, ctx, config)
     return
   end
   if not (result and result.contents) then
-    vim.notify('No information available')
+    if config.silent ~= true then
+      vim.notify('No information available')
+    end
     return
   end
   local markdown_lines = util.convert_input_to_markdown_lines(result.contents)
diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 2c6ba823db..26f0e180f5 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -1015,6 +1015,7 @@ end
 ---        - 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"
 ---@returns (table) Options
 function M.make_floating_popup_options(width, height, opts)
   validate({
@@ -1029,7 +1030,8 @@ function M.make_floating_popup_options(width, height, opts)
   local anchor = ''
   local row, col
 
-  local lines_above = vim.fn.winline() - 1
+  local lines_above = opts.relative == 'mouse' and vim.fn.getmousepos().line - 1
+    or vim.fn.winline() - 1
   local lines_below = vim.fn.winheight(0) - lines_above
 
   if lines_above < lines_below then
@@ -1042,7 +1044,9 @@ function M.make_floating_popup_options(width, height, opts)
     row = 0
   end
 
-  if vim.fn.wincol() + width + (opts.offset_x or 0) <= api.nvim_get_option('columns') then
+  local wincol = opts.relative == 'mouse' and vim.fn.getmousepos().column or vim.fn.wincol()
+
+  if wincol + width + (opts.offset_x or 0) <= api.nvim_get_option('columns') then
     anchor = anchor .. 'W'
     col = 0
   else
@@ -1062,7 +1066,7 @@ function M.make_floating_popup_options(width, height, opts)
     col = col + (opts.offset_x or 0),
     height = height,
     focusable = opts.focusable,
-    relative = 'cursor',
+    relative = opts.relative == 'mouse' and 'mouse' or 'cursor',
     row = row + (opts.offset_y or 0),
     style = 'minimal',
     width = width,
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 3fdd062ef0..f81d26b486 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -74,6 +74,7 @@
 ///      - "editor" The global editor grid
 ///      - "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 (row,col):
 ///      - "NW" northwest (default)
@@ -349,6 +350,8 @@ static bool parse_float_relative(String relative, FloatRelative *out)
     *out = kFloatRelativeWindow;
   } else if (striequal(str, "cursor")) {
     *out = kFloatRelativeCursor;
+  } else if (striequal(str, "mouse")) {
+    *out = kFloatRelativeMouse;
   } else {
     return false;
   }
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 7442e60024..40249c2f9d 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1028,10 +1028,11 @@ typedef enum {
   kFloatRelativeEditor = 0,
   kFloatRelativeWindow = 1,
   kFloatRelativeCursor = 2,
+  kFloatRelativeMouse = 3,
 } FloatRelative;
 
 EXTERN const char *const float_relative_str[] INIT(= { "editor", "win",
-                                                       "cursor" });
+                                                       "cursor", "mouse" });
 
 typedef enum {
   kWinStyleUnused = 0,
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 5c05417dd8..37f297909a 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -804,6 +804,15 @@ void win_config_float(win_T *wp, FloatConfig fconfig)
     fconfig.row += curwin->w_wrow;
     fconfig.col += curwin->w_wcol;
     fconfig.window = curwin->handle;
+  } else if (fconfig.relative == kFloatRelativeMouse) {
+    int row = mouse_row, col = mouse_col, grid = mouse_grid;
+    win_T *mouse_win = mouse_find_win(&grid, &row, &col);
+    if (mouse_win != NULL) {
+      fconfig.relative = kFloatRelativeWindow;
+      fconfig.row += row;
+      fconfig.col += col;
+      fconfig.window = mouse_win->handle;
+    }
   }
 
   bool change_external = fconfig.external != wp->w_float_config.external;
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index f0950959ff..038b9739a5 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -168,6 +168,29 @@ describe('float window', function()
     eq(7, pos[2])
   end)
 
+  it('opened with correct position relative to the mouse', function()
+    meths.input_mouse('left', 'press', '', 0, 10, 10)
+    local pos = exec_lua([[
+      local bufnr = vim.api.nvim_create_buf(false, true)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 1,
+        row = 2,
+        relative = 'mouse',
+        style = 'minimal'
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(12, pos[1])
+    eq(11, pos[2])
+  end)
+
   it('opened with correct position relative to the cursor', function()
     local pos = exec_lua([[
       local bufnr = vim.api.nvim_create_buf(false, true)