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, 14 insertions, 1 deletions

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;