backport: fix(windowing): positioning of relative floats

Fix relative floating windows so that they open in the correct position
relative to each other. Also make sure that their positions are correct
immediately after creation without a redraw.

src/nvim/window.c

@@ -737,6 +737,37 @@ void win_config_float(win_T *wp, FloatConfig fconfig)
     redraw_later(wp, NOT_VALID);
   }
 
+  // compute initial position
+  if (wp->w_float_config.relative == kFloatRelativeWindow) {
+    int row = wp->w_float_config.row;
+    int col = wp->w_float_config.col;
+    Error dummy = ERROR_INIT;
+    win_T *parent = find_window_by_handle(wp->w_float_config.window, &dummy);
+    if (parent) {
+      row += parent->w_winrow;
+      col += parent->w_wincol;
+      ScreenGrid *grid = &parent->w_grid;
+      int row_off = 0, col_off = 0;
+      screen_adjust_grid(&grid, &row_off, &col_off);
+      row += row_off;
+      col += col_off;
+    }
+    api_clear_error(&dummy);
+    if (wp->w_float_config.bufpos.lnum >= 0) {
+      pos_T pos = { wp->w_float_config.bufpos.lnum + 1,
+        wp->w_float_config.bufpos.col, 0 };
+      int trow, tcol, tcolc, tcole;
+      textpos2screenpos(wp, &pos, &trow, &tcol, &tcolc, &tcole, true);
+      row += trow - 1;
+      col += tcol - 1;
+    }
+    wp->w_winrow = row;
+    wp->w_wincol = col;
+  } else {
+    wp->w_winrow = fconfig.row;
+    wp->w_wincol = fconfig.col;
+  }
+
   // changing border style while keeping border only requires redrawing border
   if (fconfig.border) {
     wp->w_redr_border = true;
@@ -770,7 +801,6 @@ int win_fdccol_count(win_T *wp)
   }
 }
 
-
 void ui_ext_win_position(win_T *wp)
 {
   if (!wp->w_floating) {
@@ -817,6 +847,8 @@ void ui_ext_win_position(win_T *wp)
 
       int comp_row = (int)row - (south ? wp->w_height : 0);
       int comp_col = (int)col - (east ? wp->w_width : 0);
+      comp_row += grid->comp_row;
+      comp_col += grid->comp_col;
       comp_row = MAX(MIN(comp_row, Rows-wp->w_height_outer-1), 0);
       comp_col = MAX(MIN(comp_col, Columns-wp->w_width_outer), 0);
       wp->w_winrow = comp_row;

test/functional/ui/float_spec.lua

@@ -152,6 +152,132 @@ describe('float window', function()
     eq(10, width)
   end)
 
+  it('opened with correct position', function()
+    local pos = exec_lua([[
+      local bufnr = vim.api.nvim_create_buf(false, true)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 7,
+        row = 9,
+        relative = 'editor',
+        style = 'minimal'
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(9, pos[1])
+    eq(7, 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)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 7,
+        row = 9,
+        relative = 'cursor',
+        style = 'minimal'
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(9, pos[1])
+    eq(7, pos[2])
+  end)
+
+  it('opened with correct position relative to another window', function()
+    local pos = exec_lua([[
+      local bufnr = vim.api.nvim_create_buf(false, true)
+
+      local par_opts = {
+        width = 50,
+        height = 50,
+        col = 7,
+        row = 9,
+        relative = 'editor',
+        style = 'minimal'
+      }
+
+      local par_win_id = vim.api.nvim_open_win(bufnr, false, par_opts)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 7,
+        row = 9,
+        relative = 'win',
+        style = 'minimal',
+        win = par_win_id
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(18, pos[1])
+    eq(14, pos[2])
+  end)
+
+
+  it('opened with correct position relative to another relative window', function()
+    local pos = exec_lua([[
+      local bufnr = vim.api.nvim_create_buf(false, true)
+
+      local root_opts = {
+        width = 50,
+        height = 50,
+        col = 7,
+        row = 9,
+        relative = 'editor',
+        style = 'minimal'
+      }
+
+      local root_win_id = vim.api.nvim_open_win(bufnr, false, root_opts)
+
+      local par_opts = {
+        width = 20,
+        height = 20,
+        col = 2,
+        row = 3,
+        relative = 'win',
+        win = root_win_id,
+        style = 'minimal'
+      }
+
+      local par_win_id = vim.api.nvim_open_win(bufnr, false, par_opts)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 3,
+        row = 2,
+        relative = 'win',
+        win = par_win_id,
+        style = 'minimal'
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(14, pos[1])
+    eq(12, pos[2])
+  end)
+
+
   local function with_ext_multigrid(multigrid)
     local screen
     before_each(function()