fix(ui): curs_columns() scrolls a window due for a full redraw #42625

Problem:
After a resize, a decoration provider's "on_start" calling line('w0')
makes curs_columns() scroll a window whose grid offsets are stale: it
shifts cells of another window and sends a wasted grid_scroll.

Solution:
Don't scroll a window that is due for a full redraw.

src/nvim/drawscreen.c

@@ -2571,7 +2571,8 @@ void win_scroll_lines(win_T *wp, int row, int line_count)
   int checked_width = MIN(grid->cols - col, wp->w_view_width);
   int checked_height = MIN(grid->rows - row_off, wp->w_view_height);
 
-  // No lines are being moved, just draw over the entire area
+  // No lines are being moved, or the window is not on the grid (stale offsets, see curs_columns()):
+  // just draw over the entire area.
   if (checked_width <= 0 || row + abs(line_count) >= checked_height) {
     return;
   }

src/nvim/move.c

@@ -1143,7 +1143,10 @@ void curs_columns(win_T *wp, int may_scroll)
     extra = (prev_skipcol - wp->w_skipcol) / width2;
     // TODO(bfredl): this is very suspicious when not called by win_update()
     // We should not randomly alter screen state outside of update_screen() :(
-    if (wp->w_grid.target) {
+    if (wp->w_grid.target
+        // A window due for a full redraw is not scrolled: its grid offsets may be stale (e.g. after
+        // a resize, until win_grid_alloc()).
+        && wp->w_redr_type < UPD_NOT_VALID) {
       win_scroll_lines(wp, 0, extra);
     }
   } else if (!wp->w_p_sms) {

test/functional/ui/decorations_spec.lua

@@ -1414,20 +1414,33 @@ describe('decoration_providers', function()
   it('no crash when "start" scrolls a window after the screen shrinks', function()
     clear()
     local screen = Screen.new(200, 40)
-    exec_lua(function()
+    local win = exec_lua(function()
       vim.cmd('rightbelow vnew')
       local win = vim.api.nvim_get_current_win()
       vim.api.nvim_buf_set_lines(0, 0, -1, false, { 'title', ('x'):rep(57) })
       vim.api.nvim_win_set_cursor(win, { 2, 56 })
-      vim.cmd('wincmd p | vertical resize 120')
+      vim.cmd('wincmd p | vertical resize 120 | redraw')
       vim.api.nvim_set_decoration_provider(vim.api.nvim_create_namespace('ns'), {
         on_start = function()
           vim.fn.line('w0', win)
         end,
       })
+      return win
     end)
     screen:try_resize(100, 40)
-    assert_alive()
+    -- The window shrank to one column, so line('w0') in "on_start" changed its skipcol.
+    eq(
+      { 1, 19 },
+      exec_lua(function(w)
+        return { vim.api.nvim_win_get_width(w), vim.api.nvim_win_call(w, vim.fn.winsaveview).skipcol }
+      end, win)
+    )
+    screen:expect([[
+      ^                                                                                                  │{1:<}|
+      {1:~                                                                                                 }│x|*37
+      {3:[No Name]                                                                                          }{2:<}|
+                                                                                                          |
+    ]])
   end)
 end)