Fix #51739: Sliding values in add primitive redo panel not hiding cursor

Cursor is actually wrapped at the beginning with the help of function
`WM_cursor_grab_enable`. Though the `ND_MODE` event notifier added
inside  `editmode_enter/exit_ex` further calls `region_cusor_set()`.
This resets the cursor to default at the bottom of
`ED_region_cursor_set()`. Such sitution could be avoided by creating a
dummy cursor function for hud region. But issue remains when we call
redo panel from F9. As Julian suggested, add condition for `grabcursor`
to avoid changing cursor when editing buttons.

Pull Request: https://projects.blender.org/blender/blender/pulls/133529

source/blender/editors/screen/screen_edit.cc

@@ -660,6 +660,10 @@ static void region_cursor_set(wmWindow *win, bool swin_changed)
 {
   bScreen *screen = WM_window_get_active_screen(win);
 
+  /* Don't touch cursor if something else is controling it, like button handling. See #51739. */
+  if (win->grabcursor) {
+    return;
+  }
   ED_screen_areas_iter (win, screen, area) {
     LISTBASE_FOREACH (ARegion *, region, &area->regionbase) {
       if (region == screen->active_region) {